Enhanced SMTP status code · RFC 3463 · IANA
5.1.8 Sender domain cannot receive mail
The domain in the sender's address does not exist or cannot accept mail, so a bounce would have nowhere to go.
- 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).
Also seen as:
4.1.8 - Subject: X.1 · Address
About the originator or destination address: syntax or validity. The sender can usually correct it and retry.
How 5.1.8 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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5.1.8
Access denied, bad outbound sender
The sending account was blocked for sending spam, usually because it was compromised.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026