I have used Thunderbird on Windows XP, and then on Linux Mint without any major problems. I have, at the last count, something like 12 email addresses plus a lot on a spamguard setup. I'm not actually sure how it works, but of these 12 addresses, three are on two other ISP's, yet Thunderbird just sends and receives ok, I think via the main ISP.
There is a minor problem, which may be due to the spam settings i have setup in that occasionally, emails end up in the junk folder, but it's no big deal to check every few days.
Peter G. Shaw