The GPG plugins (PGP/Core, PGP/Mime and PGP/Inline) provide digital signature and encryption features:

• Checking signatures of received emails
• Decrypting received emails
• Sending signed and/or encrypted emails
• Creating new key pairs easily
• Optionally, caching your passphrase or using gpg-agent.

Every account can be configured with different defaults regarding signature and encryption, making it easy to quickly switch between sets of parameters.
Although deprecated, the PGP/Inline method of signing or encrypting emails is still available, so you can switch to it for correspondents who still have older mail user agents that cannot handle the OpenPGP/MIME standard.

GnuPG and GPGME are required, http://www.gnupg.org/

Here are a few screenshots illustrating the features:

• Creating a new key pair from an account's GPG preferences. A key pair (secret and public keys) is only necessary if you want to send signed emails or decrypt encrypted mail that you receive.
  
   
   
• An email with a verified digital signature.
  
   
   
• Decrypting an encrypted email.
  
   
   
• Various privacy preferences for an account.