(DC) [OS X] Signing and Encryption, using Apple Mail

As long as you have imported your certificate into the OS X Keychain, Apple Mail detects the certificates and uses them. 

1. When you open the Mail application after importing your certificate, a prompt will appear:

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2. By selecting “Always Allow” you are permitting Mail to use your personal signature to sign your emails. 

3. Under Options you can select the desired signing and encryption settings:

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4. Composing a new message.   If you have the public-key (certificate) for the individual(s) that you sending the email to, you can can enable one or both options:

• Sign the message, indicated by check mark or related icon.

• Encrypted the message, indicated by a padlock.  (Unlocked, will not be encrypted;  Locked, will be encrypted).
  Note: it may also be written out: "Digitially Sign Message" or "Encrypt Message".