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.

