I've found several ISPs that have a real problem with Thunderbird.
Hughes Net and ATT to name two.
While I used Thunderbird for years with my comcast connection I have now switched to Windows Live Mail as have most of my friends.
It sets itself up with no hassles, and I like the interface better now that I'm used to it.
I like the fact that it shows your e-mail message full hight on the right side of the screen instead all scrunched up at the bottom.