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SMTP codes

SMTP 5.4.4 Unable to route

The server cannot find where to deliver: the domain has no usable MX or A.

5.4.4
Class
5xx · Permanent failure
Retried?
No, the message bounces
Defined in
RFC 3463

What to do

How it appears in a bounce

550 5.4.4 Unable to route

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

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