Enhanced SMTP status code · RFC 3463 · IANA
5.7.136 Sender was not authenticated
The recipient (mailbox, group, public folder or mail user) accepts only messages from its own organisation. Only its administrator can change that.
- 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.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
How 5.7.136 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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5.7.136
Sender was not authenticated
The recipient (mailbox, group, public folder or mail user) accepts only messages from its own organisation. Only its administrator can change that.
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026