SMTP Error Codes Explained: Causes and Fixes

When an email fails to send, your mail server returns an SMTP error code. These three-digit codes tell you exactly what went wrong — and once you know what they mean, most problems are easy to fix.

How to Read SMTP Error Codes

SMTP codes follow a simple pattern:

• 2xx — Success (e.g. 250 OK)
• 4xx — Temporary failure — the server will retry
• 5xx — Permanent failure — the email cannot be delivered

Common SMTP Error Codes and Solutions

421 — Service Temporarily Unavailable

Meaning: The receiving mail server is temporarily unavailable — it may be overloaded or under maintenance.

Fix: Wait 15–30 minutes and try sending again. If the error persists, check if the recipient's domain has mail server issues.

450 — Mailbox Temporarily Unavailable

Meaning: The recipient's mailbox is temporarily unavailable — possibly over quota or the server is busy.

Fix: Try again later. If the error continues, the recipient may need to free up space in their mailbox.

451 — Requested Action Aborted

Meaning: The server encountered a local error and couldn't deliver the message. Often related to spam filtering or server-side issues.

Fix: Check if your IP is blacklisted (use mxtoolbox.com/blacklists.aspx). Also verify your SPF and DKIM records are correctly set up.

550 — Mailbox Unavailable / Blocked

Meaning: One of the most common errors. The recipient's address doesn't exist, or the receiving server has blocked your email due to spam filters or blacklisting.

Fix:

• Verify the recipient email address is correct.
• Check if your sending IP or domain is blacklisted.
• Make sure your SPF and DKIM records are configured.
• Review your email content for spam triggers.

552 — Message Size Limit Exceeded

Meaning: Your email is too large for the recipient's server to accept.

Fix: Reduce the size of attachments, compress files, or use a file sharing service (Google Drive, Dropbox) and send a link instead.

553 — Mailbox Name Invalid

Meaning: The recipient email address has an invalid format.

Fix: Double-check the email address for typos — especially the domain part after the @ sign.

554 — Transaction Failed / Message Rejected

Meaning: A catch-all permanent error. The message was rejected, often because your IP or domain is on a spam blacklist, or because of policy violations.

Fix:

• Check your IP reputation at mxtoolbox.com/blacklists.aspx
• Set up SPF, DKIM, and DMARC records for your domain
• Contact your email provider if the issue persists

Quick Reference Table