Importing a pfx file into Apple Mail
1. Import the pfx into your Keychain
Double-click the pfx file. This will open the Keychain Access application which will try to import the certificate and private key. If the Keychain Access application is not opened, drag the pfx file onto the Keychain Access application icon (/Applications/Utilities).2. Enter the pfx password
You are now asked for the password of the password protected PFX file. This is the password that was given to you via SMS Text message or in some other way.3. Trust the certificate by clicking "Always trust"
The pfx file not only contains the end-user certificate and private key but also the root and intermediate certificate. The Keychain Access will also try to import the root and intermediate certificate and asks whether you trust the root certificate. You should select "Always trust"1.
4. Restart Apple Mail
After importing the certificate you should restart Apple Mail.5. Finished.
Your system is now ready to send and receive encrypted email.[The following steps are optional]
We will explain how to receive and send encrypted email.
Receiving signed and encrypted email
A signed and encrypted message looks as follows:
shows that the message was encrypted and
shows that the message was signed.
Sending signed and encrypted email
You can sign and encrypt a message in the compose window by selecting the sign and encrypt options.
Encrypt: 
Sign: 