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.

S/MIME plugin handles S/MIME signed and/or encrypted mails.

WWW: http://claws-mail.org/plugin.php?plugin=gpg