Enhanced status code
A three-part code such as 5.1.1 or 4.7.0 (RFC 3463) that accompanies an SMTP reply code and gives the reason more precisely: class, subject and detail.
What it means
The first number is the class: 2 success, 4 persistent transient failure, 5 permanent failure. The second is the subject: 0 other, 1 addressing, 2 mailbox, 3 mail system, 4 network and routing, 5 mail delivery protocol, 6 message content, 7 security or policy. The third is the detail within that subject, so 5.1.1 is a bad destination mailbox, 4.2.2 a full mailbox and 5.7.1 delivery not authorised. IANA keeps the registry, which later RFCs have extended, for example with 5.7.26 for multiple failed authentication checks.
Servers that announce ENHANCEDSTATUSCODES (RFC 2034) put the code at the start of their reply text, and delivery status notifications carry it in their Status field. Since a basic 550 can mean many things, the enhanced code is usually the more useful half of a bounce, and large providers add their own codes beyond the registry, such as Microsoft's 5.7.708.
Example
550 5.7.1 Message rejected due to local policyRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes