MailCloak for Mail Clients allows Mozilla Thunderbird users to send secure, encrypted email to their contacts.

• Mozilla Thunderbird 2.0
  And more email clients

With MailCloak for Mail Clients, you can send encrypted email from Thunderbird to other MailCloak users, or to PGP and GnuPG users on any platform – because MailCloak uses GnuPG – Secure, Open Source encryption.