An internal user can send encrypted emails successfully to all recipients, except one.

The validity dates of the external recipient's private key do not match the SSH keys with which the internal user is accessing the system.

The external recipient has an expired public/private key pair that has not been revoked by the CA.

The internal user's company email servers have an incorrect implementation of OCSP and CRL settings.

The external recipient's email address and the email address associated with the external recipient's public key are mismatched.