SMTP (Simple Mail Transfer Protocol) errors occur during the email sending process. These errors often include status codes and messages that help diagnose the problem. Below is a list of the most common SMTP errors and their meanings.
Common SMTP Errors
- DNS Error 5.7.26 (Authentication Error)
- Error 550 (Requested Action Not Taken – Mailbox Unavailable)
- Error 554 (Transaction Failed)
- Error 4xx (Temporary Failure)
- Error 553 (Mailbox Name Not Allowed)
- Error 451 (Local Policy Violation or Greylisting)
- Error 552 (Storage Limit Exceeded)
- Error 530 (Authentication Required)
- Error 5.7.1 (Relay Access Denied)
- Error 5.1.1 (User Unknown)
DNS Error 5.7.26 (Authentication Error)
This error occurs when an email fails due to sender domain authentication issues (e.g., SPF, DKIM, or DMARC). The recipient's server cannot verify the domain.
How to fix:
- Ensure your SPF record is correctly configured.
- Set up a DKIM signature.
- Implement a DMARC policy with proper alignment.
Error 550 (Requested Action Not Taken – Mailbox Unavailable)
This error typically indicates that the recipient's email address does not exist, their mailbox is unavailable, or the server rejected the email due to policy violations.
How to fix:
- Verify the recipient's email address.
- Check for typos in the email address.
- Contact the recipient to confirm their mailbox is active.
Error 554 (Transaction Failed)
A general error indicating the email was rejected by the recipient's server due to reasons like blacklisting or spam content.
How to fix:
- Check if your domain or IP is blacklisted.
- Review your email content to comply with spam guidelines.
- Use authentication protocols like SPF, DKIM, and DMARC.
Error 4xx (Temporary Failure)
Temporary errors (e.g., 421 or 450) often occur because the recipient's server is busy or unavailable.
How to fix:
- Wait for the server to retry automatically.
- Ensure your server settings comply with the recipient’s policies.
Error 553 (Mailbox Name Not Allowed)
Indicates that the email address in the "From" field is not correctly formatted or allowed by the recipient's server.
How to fix:
- Ensure the "From" address is a valid email.
- Check your DNS settings for SPF compliance.
Error 451 (Local Policy Violation or Greylisting)
Often caused by greylisting, where the recipient's server temporarily rejects the email to test if the sending server retries.
How to fix:
- Wait for the server to retry.
- Ensure your email server complies with greylisting policies.
Error 552 (Storage Limit Exceeded)
Indicates the recipient’s mailbox has exceeded its storage quota and cannot receive new emails.
How to fix:
- Contact the recipient and inform them about the issue.
- Retry sending the email after the recipient clears space in their mailbox.
Error 530 (Authentication Required)
Occurs when the sending server requires authentication but was not provided, or the credentials are incorrect.
How to fix:
- Verify your email client settings for correct credentials.
- Enable SMTP authentication in your email client.
Error 5.7.1 (Relay Access Denied)
This error occurs when the email server does not allow relaying emails to external domains due to improper authentication.
How to fix:
- Ensure the outgoing server (SMTP) has proper authentication enabled.
- Verify the sender's authorization to send emails through the server.
Error 5.1.1 (User Unknown)
Indicates the recipient's email address does not exist on the server.
How to fix:
- Double-check the recipient's email address for typos.
- Verify with the recipient if the address is still active.
Understanding and addressing these SMTP errors will help ensure smooth email delivery and maintain your email reputation. If issues persist, consult your email service provider for additional assistance.