Captin

Thunderbird can store several SMTP accounts and when you reply to an email it will be sent through the account you recieved the original email.

IE:

Email from Johnny recieved on account 1, reply sent back on account 1, using account 1's reply address.
Email from Mary recieved on account 2 reply sent back on account 2, using account 2's reply address.

But you can also send emails using any of the accounts you have setup, so you could reply to an email recieved on account 1 and have the reply go out on account 2.

Each account has seperate details, like smtp server, pop server, username & password and reply address.

It will also setup seperate locations on your PC for each account, so you will get a seperate inbox, outbox etc... for each account.

I personally have seven seperate accounts running in thunderbird. And using a little program called birdiesync can use thunderbird to sync with my windows mobile phone.