Problems during encryption of mail

Børge Holse Hello again and sorry for the late reply. Yes, I did mean that. The contact has to send a signed email message to your user to have him/her save the contact and the cert/public key for the encryption to work. But I get the feeling you already know this.

Another thing that could be causing the issue, as you've done the process all over again, could be that when you send an encrypted email to a recipient the recipient's certificate/public key is used to encrypt it. If the email address of the contact doesn't match the email address in the recipient's certificate, you'll also get the message you attached.

I just did a search and found a page that actually describes it. Don't be alarmed by the old "applies to", it should still be possible if you use the appropriate key.

https://support.microsoft.com/en-us/help/276597/how-to-turn-off-e-mail-matching-for-certificates-in-outlook

Hope it helps!