Enhanced status code

SMTP error X.7.3

Security conversion required but impossible

Updated Jul 1, 2026

The short answer

Security conversion required but impossible Common cause: Secure-message conversion cannot be performed

Quick fix

Align encryption / security formats between systems

This code is written X.7.3 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

Align encryption / security formats between systems

Related codes

X.7.0 Other or undefined security status X.7.1 Delivery not authorized X.7.2 Mailing list expansion prohibited X.7.4 Security features not supported X.7.5 Cryptographic failure X.7.6 Cryptographic algorithm not supported
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