SMTP error codes, explained and fixed

The first digit is the verdict. A 5xx code is a permanent failure: the receiving server refused the message and it will not be retried; you have to fix the cause and re-send. A 4xx code is a temporary failure: the message is deferred and the sending server keeps retrying automatically, typically for 24–72 hours before it gives up and bounces.

That's the enhanced status code (RFC 3463). The first digit repeats the class (5 = permanent, 4 = temporary), the second names the subject area (7 = security or policy, 1 = addressing, 2 = mailbox), and the third pinpoints the specific condition. It's usually more precise than the three-digit reply code: 550 alone says 'refused', while 5.7.26 says 'refused because the mail was unauthenticated'.

Read the enhanced code and the text. Authentication and policy codes (5.7.x) almost always point at the sender: SPF, DKIM, DMARC, or reputation. Addressing and mailbox codes (5.1.1 nonexistent user, 5.2.2 mailbox full) point at the recipient. Temporary 4xx deferrals can be either: sender rate limits and reputation throttles, or a receiving server having a bad day.

Providers write their own human-readable text after the numeric codes, and some add proprietary sub-codes (Microsoft's 5.7.509, for example). The numbers are the standard part: match the reply code and enhanced status code first, then use the provider's wording to identify the exact variant.