Enhanced SMTP status code · RFC 3463 · IANA
5.2.4 Mailing list expansion problem
The address is a mailing list that could not be expanded to its members.
- Class: Permanent failure
Class 5: a permanent failure. Resending the same message will not help: something about the message, the sender or the destination must change (RFC 3463 §3.1). The registry says this code can be temporary or permanent.
Also seen as:
4.2.4 - Subject: X.2 · Mailbox
Something about the recipient's mailbox; under the recipient's control.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026