I've been using Thunderbird for my email for a very long time. Probably since some early 1.0 release.

In these years, I've also had it on Windows and Linux, I've migrated it easily across many OS installs and hardware changes, I've used it with different kinds of email accounts and servers. It's worked with PGP encrypted mail, with SpamAssassin on the server and more.

It's great. It doesn't change much, which is probably a good thing, Firefox lost me as a user at some point. Thunderbird mostly stays the same, adding features occasionally. As I write this, I realize I'm so used to Thunderbird I'm not even sure what other clients are available. Definitely one of the best programs I've used.