How to use gmail to filter spam
1) forward your mail to your gmail account.
2) send mail from thunderbird or your favourite mail client, as usual.
3) Train the gmail spam filter by reporting as spam
4) receive mail via the gmail interface, and send mail via gmail if its ok for you to send that way (remember if you send from gmail it will appears as gmail)
5) filter all legit mail and on filter, send to another mail address to open with thunderbird.
6) when you receive the mail in that client or other, make sure your sending address is the same as the address that is forwarding to gmail.
you can log in to gmail once every day to check its doing its job ok 
