I use the same PowerBook at home and at work. Due to firewall issues, I can't access my home e-mail ISP from work and at home, I can't access my work e-mail. Thus in Mail.app, I always had to use the "Get mail from XXXX" menu rather than the more convenient "Get Mail" button, which would try to collect from both places and get into a long time-out every time.
In Panther, a new Mail.app menu items allow you to (easily) toggle which sets of e-mail boxes you want the Get Mail button to use. To remove an account from collection, just go to the menu Mailbox -> Online Status and take the accounts you can't reach offline.
Actually it's simpler than that! Just press the Get Mail button with all the accounts enabled (Mailbox -> Go Online). After Mail tries once and fails to reach an account, it will take it offline for you and display a small lightning bolt next to the InBox for that account. Clicking that lightning bolt will restore the account to being collected the next time you press Get Mail.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031108111211991