Enhanced SMTP status code · RFC 3463 · IANA
5.7.27 Sender address has a null MX
The sender's domain publishes a null MX, so it could not receive a bounce, and the recipient refuses such senders (RFC 7505).
- 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:
4.7.27 - Subject: X.7 · Security and policy
Policies (per-host or per-recipient filters) and cryptographic operations; up to the sender, the recipient or both.
- Associated basic codes (IANA registry)
How 5.7.27 appears at providers
The exact text from the provider's documentation, as you see it in the bounce, and what it means.
Gmail
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550 5.7.27
This message was blocked because it didn't pass SPF authentication. Gmail requires bulk email senders to authenticate their email with SPF.
The message does not pass SPF, which Gmail requires from bulk senders. Check with SPF generator.
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421 4.7.27
Your email has been rate limited because SPF authentication didn't pass for this message. Gmail requires all bulk email senders to authenticate their email with SPF. Authentication results: SPF domain-name with IP address: ip-address = Did not pass.
The message does not pass SPF, which Gmail requires from bulk senders. Check with SPF generator.
What you can check here
Have the whole error message?
Sources: RFC 5321 · RFC 3463 · RFC 3464 · IANA · Gmail · Microsoft 365 · read on 26 September 2026