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SMTP codes

SMTP 521 Server does not accept mail

This host never accepts mail (RFC 7504), for example a domain with a null MX.

521
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 7504

What to do

How it appears in a bounce

521 Server does not accept mail

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

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