Enhanced SMTP status code · RFC 3463 · IANA
5.1.6 Mailbox moved, no forwarding address
The address was valid once but no longer accepts mail, and there is no new address to forward to.
- 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 is only useful as a permanent failure.
- Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
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Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026