If you have as many email accounts as I do, Thunderbird will get rather cluttered.
I suggest
Pegasus Mail. It's powerful and safe and ideal for people who have multiple email accounts. You can specify even more identity-specific settings in it than Thunderbird, and in multi-user mode you can micro-manage even further.
Oh, and when you go to create a mail folder, you can choose to store email in unix mbox format, which is widely supported and for which conVersion tools are readily available.
There's also a PGP plugin available that makes using PGP with Pegasus a snap.