What’s a “bounce signature” and why is it useful?

A bounce signature uniquely identifies a bounce type:

What comprises a signature:

SMTP response code. Enhanced status code. Key phrases in diagnostic text. Provider or MTA identifier.

Signature examples:

Gmail user unknown: 550 5.1.1 + "does not exist" + mx.google.com. Microsoft rate limit: 421 + RP-001 + outlook.com MX. Generic full mailbox: 552 5.2.2 + "mailbox full."

Why signatures are useful:

Consistent classification across occurrences. Machine-readable categorization. Dictionary building and maintenance. Pattern matching automation.

Building a signature library:

Collect bounce samples. Extract key identifying elements. Create matching rules. Test and refine.

Signatures are fingerprints for bounces. They enable reliable identification and handling.