Enhanced status code

SMTP error X.6.3

Conversion required but unsupported

Updated Jul 1, 2026

The short answer

Conversion required but unsupported Common cause: Gateway cannot perform required message conversion

Quick fix

Send using a more compatible format

This code is written X.6.3 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

Send using a more compatible format

Related codes

X.6.0 Other or undefined media error X.6.1 Media not supported X.6.2 Conversion required and prohibited X.6.4 Conversion with loss performed X.6.5 Conversion failed X.6.6 Message content not available
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