Multi-platform email application with mail encryption through the Enigmail add-on.
Thunderbird does not encrypt your email by default. Here is a guide by Security In A Box to encrypting your email with Mozilla Thunderbird, GNU Privacy Guard (GPG), and Enigmail.
Thunderbird can optionally connect over Tor with the TorBirdy addon.
While Thunderbird can be used as a chat and IRC client, it does not offer encrypted messaging.