This guide describes how to integrate a personal certificate for signing and encrypting emails in Apple Mail for macOS. To do this, you need a certificate, which must be available as a .p12 file.
How do I request a personal certificate?

Please note the information on Email Encryption.
 

1. First you need to import your certificate with your private key into the macOS keychain. To do this, click on the file (PKCS12 format) that contains the key and the certificate.

   

2. Select "login" for keychain and click "Add".

   

3. When prompted for the certificate password, enter the password and click "OK".

   

4. After the installation you will be redirected to the Keychain Access. There you can see the certificate you just installed under "My Certificates".

   

5.  Now open the "Settings" of Apple Mail.

   

6. Click on "Server settings" under "Accounts" and select "Advanced Exchange settings".

   

7. Select your certificate under TLS certificate and confirm with "OK". 

   

8. Please "save" the settings.

   

9. When you now send an email, you can choose whether you want to encrypt it, provided the recipient also has a certificate. You can change this by clicking on the lock symbol.