Enhanced status code

SMTP error X.1.8

Bad sender system address

Updated Jul 1, 2026

The short answer

Bad sender system address Common cause: Sender domain is invalid or cannot accept return mail

Quick fix

Check sender-domain DNS and address configuration

This code is written X.1.8 because the reason is the same whether the failure is temporary (a 4 first digit) or permanent (a 5). Read the first digit of your own bounce to know which applies.

How to fix it

Check sender-domain DNS and address configuration

Related codes

X.1.0 Other address status X.1.1 Bad destination mailbox address X.1.2 Bad destination system address X.1.3 Bad destination mailbox address syntax X.1.4 Destination mailbox address ambiguous X.1.5 Destination address valid
All SMTP error codes
Ready when you are

Most rejections come back to sender reputation.

Blocklists, auth failures, and policy blocks are what warmup prevents. Warm up your domain so your mail is trusted before you hit send.

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