A short guide to the numbers in bounce messages.
THE FIRST DIGIT
4xx: a temporary failure. The sending server will retry, usually for several days. 5xx: a permanent failure. No retry will be made.
COMMON CODES
421: service unavailable, try later. Usually a busy or rate-limiting server. 450 and 451: temporary rejection, often greylisting. Delivery succeeds on retry. 452: insufficient storage, usually a full mailbox at the recipient's end. 535: authentication failed. Your credentials were refused. 550: mailbox unavailable. Either the address does not exist, or the message was rejected on policy grounds. 552: message too large, or mailbox over quota. 554: transaction failed, frequently a spam rejection.
READING BEYOND THE NUMBER
The text after the code matters more than the code itself. It usually names the cause and sometimes links to the blocking organisation's explanation page.
WHAT TO SEND US
If you open a ticket about a bounce, include the full bounce message rather than a description. The code and text together identify the cause immediately.
