Enhanced status code

SMTP error X.7.6

Cryptographic algorithm not supported

Updated Jul 1, 2026

The short answer

Cryptographic algorithm not supported Common cause: Receiver does not support required algorithm

Quick fix

Use a mutually supported cryptographic algorithm

This code is written X.7.6 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

Use a mutually supported cryptographic algorithm

Related codes

X.7.0 Other or undefined security status X.7.1 Delivery not authorized X.7.2 Mailing list expansion prohibited X.7.3 Security conversion required but impossible X.7.4 Security features not supported X.7.5 Cryptographic failure
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