Where can I find a list of SMTP error codes?
Still have a question, spotted an error, or have a better explanation or a source we should cite?
You just got a bounce code like '4.2.1' or '5.1.1,' and you have no idea what it means. So where do you actually look?
SMTP error codes come from international standards, but ESPs often customize them. So you'll need two sources of truth.
Start with the standards: RFC 5321 defines the foundational SMTP protocol and reply codes (those 2xx, 3xx, 4xx, 5xx numbers). RFC 3463 adds the enhanced format (the X.Y.Z pattern you're probably seeing). These are authoritative, but they're dense reading.
Then check your ESP: SendGrid, Mailchimp, Postmark, and others publish bounce guides specific to their platforms. Your ESP's code 4.2.1 might mean something slightly different than Google's, so look at your provider's docs first.
Check the mailbox provider: Gmail and Outlook publish postmaster guidelines. Yahoo Mail does too. These show how each inbox interprets SMTP codes.
Here's the fastest path: ask your ESP for a bounce classification guide. Most have one. If they don't, grab the RFC and cross-reference with their API documentation. You can also paste your bounce code into Review My Emails' Email Header Analyzer to decode it quickly.
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