Enhanced status code

SMTP error X.1.3

Bad destination mailbox address syntax

Updated Jul 1, 2026

The short answer

Bad destination mailbox address syntax Common cause: Recipient address is malformed

Quick fix

Correct the address format

This code is written X.1.3 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

Correct the address format

Related codes

X.1.0 Other address status X.1.1 Bad destination mailbox address X.1.2 Bad destination system address X.1.4 Destination mailbox address ambiguous X.1.5 Destination address valid X.1.6 Destination mailbox moved, no forwarding address
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.

Start 7-day free trial Book a demo
7-day free trialNo credit cardCancel anytime