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.

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

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    Screenshot macOS: installation window
  3. When prompted for the certificate password, enter the password and click "OK".

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

    explanatory screenshot to the previous description with marker on the certificate
    Screenshot macOS: Keychain Access
  5.  Now open the "Settings" of Apple Mail.
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    Screenshot Apple Mail: Settings
  6. Click on "Server settings" under "Accounts" and select "Advanced Exchange settings".
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    Screenshot Apple Mail: Account settings
  7. Select your certificate under TLS certificate and confirm with "OK". 
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    Screenshot Apple Mail: Certificate selection
  8. Please "save" the settings.
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    Screenshot Apple Mail: Account 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.
    explanatory screenshot to the previous description
    Screenshot Apple Mail: new e-mail