This guide breaks down the common errors you might hit when sending emails through Concours Pro. Each error comes with a specific SMTP response, including a Category, Error Code, and Status Code. We’ve laid out clear explanations of what these codes mean, why the issue might be happening, and practical steps to fix them fast. Use this as your go-to resource to sort out any email delivery hiccups.
Contents
- Blocked Emails
- DMARC Failures
- Invalid Email Addresses
- No Mailbox Available
- Mailbox Full
- Rate Limit Issues
- Recipient Server Errors
- Rejection Errors
- Fixing Sender Reputation Issues
Blocked Emails
Your email got blocked due to a policy or rule set by the recipient’s system. This could be down to mailbox quotas, email size limits, or specific organisational restrictions.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Blocked | 554 | 5.2.3 | Your email was blocked because of a policy or rule, like mailbox quotas, email size limits, or organisational restrictions preventing delivery. |
DMARC Failures
The email failed DMARC authentication because the From header doesn’t match the SPF or DKIM-validated domains.
Fix It:
- Make sure the From address aligns with your sending domain.
- Double-check that SPF and DKIM are properly set up for your domain.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| DMARC | 509 | 5.7.509 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.0.0 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.20 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.26 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.5 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.509 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.6 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.8 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | 5.7.9 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 550 | Missing | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 553 | – | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 554 | 5.0.0 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 554 | 5.7.5 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
| DMARC | 555 | 5.7.5 | Email rejected by the recipient’s provider for failing DMARC. Ensure the From header domain matches SPF/DKIM-validated domains. |
Invalid Email Addresses
This error means you tried sending to an email address that doesn’t exist. The contact might’ve given a dodgy address or made a typo.
Fix It:
- Use Email Validation to check email addresses before sending.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Invalid address | 450 | 4.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 500 | 5.0.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 500 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 501 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 501 | 5.1.3 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 501 | 5.5.4 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 501 | 5.7.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 505 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 508 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 511 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 511 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 521 | 5.3.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 522 | 5.7.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 525 | 5.7.13 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 540 | 5.7.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 542 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.0.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.0.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.0.350 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.10 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.181 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.351 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.1.6 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.2.1 | The email account you tried to reach is inactive. |
| Invalid address | 550 | 5.2.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.2.14 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.2.5 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.3.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.3.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.4.310 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.4.312 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.4.350 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.4.355 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.5.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.5.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.5.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.7.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.7.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.7.100 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.7.13 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | 5.8.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | – | The recipient’s email address is invalid or doesn’t exist and is already on the bounce list. Revalidate the email and try again. |
| Invalid address | 550 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 550 | – | Errors caused by misconfiguration or outage on the recipient’s MX server. |
| Invalid address | 551 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 551 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 552 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.1.3 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.3.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.5.3 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.7.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 553 | 5.7.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.0.0 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.1.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.2.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.2.2 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.5.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.7.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | 5.7.7 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 554 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 556 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 572 | – | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 577 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 585 | 5.1.1 | The recipient’s email address is invalid or doesn’t exist. |
| Invalid address | 605 | – | The recipient’s email address is already on the bounce list. Revalidate the email and try again. |
| Invalid address | 612 | – | Errors caused by misconfiguration or outage on the recipient’s MX server. |
No Mailbox Available
This error pops up when you try sending to an address that doesn’t have a mailbox set up.
Fix It:
- Use Email Validation to confirm email addresses are legit.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Mailbox | 450 | 4.2.2 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 452 | – | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 498 | – | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 511 | – | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 516 | 5.1.6 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 522 | 5.2.2 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 4.2.2 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.0.350 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.2.0 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.2.2 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.4.6 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.5.0 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 550 | 5.5.1 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 552 | 5.1.1 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 552 | 5.2.1 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 552 | 5.5.2 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 552 | – | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 553 | 5.3.1 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
| Mailbox | 554 | 5.1.1 | The recipient’s domain has no mailbox and doesn’t accept mail. Send to a valid email address. |
Mailbox Full
A Mailbox Full error happens when the recipient’s inbox is maxed out and can’t take new emails. It’s usually a temporary issue—once they clear some space or their mailbox limit is increased, your email should go through.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Mailbox Full | 452 | 4.1.1 | The recipient’s mailbox is full and can’t accept new emails. |
| Mailbox Full | 452 | 4.2.2 | The recipient’s mailbox is full and can’t accept new emails. |
| Mailbox Full | 550 | 5.2.2 | The recipient’s mailbox is full and can’t accept new emails. |
| Mailbox Full | 552 | 5.2.2 | The recipient’s mailbox is full and can’t accept new emails. |
| Mailbox Full | 552 | – | The recipient’s mailbox is full and can’t accept new emails. |
Rate Limit Issues
The recipient’s server blocked your email because you’re sending too many emails too quickly.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Rate Limit | 221 | – | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
| Rate Limit | 550 | 4.2.1 | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
| Rate Limit | 550 | 5.2.1 | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
| Rate Limit | 550 | 5.2.122 | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
| Rate Limit | 554 | 5.2.121 | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
| Rate Limit | 554 | 5.2.122 | The recipient’s server denied your email due to sending too many emails too fast. Contact the recipient another way. |
Recipient Server Errors
The recipient’s server blocked your email because it restricts communication from outside its domain or due to specific server issues.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Recipient Error | 450 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 502 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 503 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 510 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 521 | 5.3.0 | Cloudflare’s email routing service flagged an issue with email routing or processing. |
| Recipient Error | 530 | 5.3.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 530 | 5.7.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 530 | 5.7.1 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 530 | 5.7.57 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 530 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 535 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 541 | 5.4.1 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 4.4.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 4.4.2 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 4.4.7 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.1.3 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.1.9 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.3.5 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.103 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.108 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.142 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.300 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.315 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.316 | The mail server couldn’t deliver the message after multiple attempts and it expired. This happens when the recipient’s server refuses the connection or is unreachable for too long. |
| Recipient Error | 550 | 5.4.317 | The mail server couldn’t deliver the message after multiple attempts and it expired. This happens when the recipient’s server refuses the connection or is unreachable for too long. |
| Recipient Error | 550 | 5.4.4 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.4.7 | The email server tried delivering the message for too long but couldn’t within the time limit. The email is now permanently rejected. |
| Recipient Error | 550 | 5.5.5 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.105 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.124 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.128 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.129 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.133 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.134 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.136 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.193 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.230 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.367 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.51 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.520 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.54 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.64 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.703 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.705 | The recipient’s Microsoft 365 or Office 365 tenant hit a limit or threshold, blocking further email deliveries. This could be due to message volume, inbound limits, or security triggers. |
| Recipient Error | 550 | 5.7.900 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.902 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.929 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.987 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.7.999 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 550 | 5.8.2 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 551 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 552 | 5.7.1 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 553 | 5.0.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 553 | 5.1.8 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 4.3.5 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.1.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.1.2 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.2.0 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.4.6 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.5.4 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.7.1 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 554 | 5.7.105 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 555 | 5.7.1 | The email platform isn’t supported for communication with Concours Pro’s systems. This could be due to unsupported email clients or formats. |
| Recipient Error | 556 | 5.7.1 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 556 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 571 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 577 | 5.7.7 | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 579 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
| Recipient Error | 701 | – | The recipient’s server denied your email. External communication is restricted. Contact the recipient another way. |
Rejection Errors
These errors occur when the sending domain lacks valid DNS records, the email is too large, or there’s a mail loop issue.
| Category | Code | Enhanced Code | Definition |
|---|---|---|---|
| Rejection | 450 | 4.1.8 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 501 | 5.1.0 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 501 | 5.1.7 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 501 | 5.1.8 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 501 | 5.2.0 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 501 | NA | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 533 | 5.7.1 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.1.0 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.1.7 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.1.8 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.2.3 | The email is too large for the recipient’s size limit. |
| Rejection | 550 | 5.4.0 | The email bounced due to exceeding the maximum number of hops between mail servers, often caused by a mail loop. |
| Rejection | 550 | 5.4.12 | The email bounced due to exceeding the maximum number of hops between mail servers, often caused by a mail loop. |
| Rejection | 550 | 5.4.14 | The email bounced due to exceeding the maximum number of hops between mail servers, often caused by a mail loop. |
| Rejection | 550 | 5.4.6 | The email was blocked by the recipient’s provider due to a detected mail loop. |
| Rejection | 550 | 5.5.2 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.13 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.21 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.23 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.24 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.26 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.27 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.368 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 550 | 5.7.57 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 553 | 5.1.8 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 554 | 5.1.8 | The sending domain lacks valid DNS records. Verify the domain and add necessary DNS records with your provider. |
| Rejection | 554 | NA | Possible reasons: invalid domain in the MAIL FROM or poor sending domain reputation. |
Fixing Sender Reputation Issues
How to Fix: Email Blocked for Potential Spam or Malicious Activity
1. Check IP and Domain Blacklists
- Use tools like Blacklist Check.
- If your IP or domain is listed, follow the blacklist service’s delisting process.
2. Ensure Proper DNS Configuration
- SPF Record: Confirms which servers can send emails for your domain.
- DKIM (DomainKeys Identified Mail): Adds a digital signature to verify email authenticity.
- DMARC (Domain-based Message Authentication, Reporting, and Conformance): Sets policies for handling emails that fail SPF/DKIM checks.
Test DNS Records:
- Use MXToolbox to validate SPF, DKIM, and DMARC records.
3. Monitor and Improve Email Reputation
- Microsoft SNDS: Check your IP reputation with Microsoft (Outlook, Hotmail).
- Google Postmaster Tools: See how Gmail handles your emails.
4. Send Smaller Email Batches to Build Reputation
- Don’t blast a heap of emails from a new or cold IP/domain. Start small and ramp up slowly to build trust.
5. Clean and Segment Email Lists
- Remove inactive or invalid addresses to cut down on bounces.
- Use Email Validation tools to scrub your lists before campaigns.
- Keep an eye on complaints or spam reports and remove problematic recipients.
6. Check Email Content and Follow Best Practices
- Steer clear of spammy subject lines like “You’ve Won!!!” or “Urgent!”.
- Keep a good text-to-image ratio—don’t send emails that are just images.
- Ensure your links are safe and not blacklisted.
- Always include an unsubscribe link in marketing emails to stay compliant with anti-spam laws.
For more information on email best practices, read these related articles:
An Introduction to Email Deliverability
Email Sending Guide: Best Practices & Warm-Up for Concours Pro Users
