Bounce reader

How it decides

• The first digit of the reply code says how final it is: 4 is temporary, 5 is permanent (RFC 5321). "Soft" and "hard" bounce are the everyday names for the same two.
• The middle number of the enhanced code says what it's about (RFC 3463): 1 the address, 2 the mailbox, 3 the mail system, 4 the network, 5 the protocol, 6 the content, 7 security or policy.
• Permanent and about the address (5.1.x, 5.2.1): stop mailing it. One is enough.
• Permanent and about security, policy or content (5.7.x, 5.6.x): the server refused the sender or the message. The address is fine; suppressing it loses a real person.
• Mailbox full (x.2.2): count it, and stop after three in a row over two weeks or more. That number is ours, not a standard.
• The words can overrule the numbers. A 5xx that says "rate" or "try again later" is read as temporary. A policy code that says "user unknown" is handed to you to judge.
• A bare code with no enhanced part (550 No such user here) is decided by the words alone, so the answer is less certain.

Providers write their own text, and some use codes loosely. Treat the result as a first reading. When it says "a person should read this one", it means it.

The reasoning behind each rule, with a table and a small function for your own bounce handler: A bounce is a to-do item: hard and soft bounces, and when to stop mailing an address.