What is an SMTP enhanced status code?
Also called: SMTP reply code, DSN code, bounce code
An enhanced status code is the three-part number, such as 5.1.1, that follows the basic three-digit SMTP reply code and says more precisely why mail was accepted, deferred or refused. Defined in RFC 3463, the first digit gives the class: 2 for success, 4 for a temporary failure, 5 for a permanent one.
The basic reply code, such as 550, is the part every server must send. The enhanced code adds a subject and a detail: in 5.1.1 the middle 1 means an addressing problem and the last 1 means the mailbox does not exist. In 5.7.1 the 7 means a security or policy reason.
Mailbox providers add their own detail on top. Gmail and Microsoft 365 use codes like 5.7.26 and 5.7.515 for their own authentication rules, and the text after the code often names the exact cause and links to the provider's help page.
For list cleaning, the class digit is what matters: a 5 on a mailbox means remove it, a 4 means try again later. For deliverability, the 5.7.x codes matter most, because they mean the problem is you rather than the address.
Example
550 5.1.1 The email account that you tried to reach does not exist. → permanent, addressing, bad mailbox.
421 4.7.0 Try again later, closing connection. → temporary, policy.How SimpleVerifier handles it
During a mailbox probe, SimpleVerifier reads the reply code: 2xx is valid, a permanent 5xx on the mailbox is invalid, and a temporary 4xx is risky · Server would not confirm.
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Sources
By SimpleVerifier Team · Last reviewed 2026-10-13 · All 50 glossary terms