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SMTP codes

SMTP 5.1.8 Bad sender address

The sender address or its domain is rejected: the domain does not resolve or has no MX/A.

5.1.8
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 3463

What to do

How it appears in a bounce

550 5.1.8 Bad sender address

Paste the full bounce into the decoder — it finds every code and recipient and says what to check.

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