presumably they don't support Thunderbird because they don't want to train their script monkeys in yet another mail package.
There are plenty of settings around POP3 accounts that might be different, have you double checked things are identical ....
"Make sure that the incoming and outgoing mail server addresses are.
Incoming Mail (POP3): mail.madasafish.com
Outgoing Mail (SMTP): mail.madasafish.com
Confirm that the entry for Account name contains your email address and that Password contains the password you use to connect to Madasafish (this is case sensitive, and the username must be in lowercase).
Do not tick Log on using Secure Password Authentication.
My server requires authentication does not need to be ticked unless you are not connected to Madasafish."
Phil
MaxDSL - goes as fast as it can and doesn't read the line checker first.
MaxDSL diagnostics
Are your kids pirates ? Limewire, Bearshare, Kazaa, BitTorrent, eMule are all tools of the trade.