For each mail account under Mail Accounts, click on the account name and click Edit. Go to the Receiving Mail pane. Change the host to 127.0.0.1, and the username to host:username. For example, if the host is furble.mail.com, and the username is jrbobdobbs, then the new host will be 127.0.0.1, and the new username will be furble.mail.com:jrbobdobbs. When this is complete, click OK.