Enhanced SMTP status code · RFC 3463 · IANA
5.1.0 Address problem
Something about an address in the message caused the error, with no further detail.
- 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).
- Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
How 5.1.0 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Microsoft 365 / Exchange Online
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5.1.0
Sender denied
The sender's address could not be found; it usually happens when replying from a message saved and opened offline in Outlook.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026