In the current version of Mozilla Thunderbird (3.1.2), there is a considerable amount of auto configuration checks done during the account creation process. These checks will default to the best options feasible for your mail server, such as using SSL over non SSL and to some users preference IMAP over POP3. For servers that offer both IMAP and POP as protocol options, IMAP will be chosen by default. Users who want to use POP3 instead of IMAP may find this auto configuration a challenge to work around. To use the mail client auto config and have POP selected do the following to create a new account:
Incoming : mail.example.com IMAP 143 STARTTLS Outgoing : mail.example.com SMTP 25 STARTTLS