Apple Mail checks Exchange Public Folders for new messages
Apple Mail in Mac OS X 10.3 "Panther" can attempt to index and check the unread count for every Public Folder. This can happen whether you have Apple Mail set up to use the Microsoft Exchange account type or the IMAP account type.
We do recommend that you upgrade to Panther if you wish to use Apple Mail with your RIT Exchange account. Its version of Apple Mail provides some direct support for Exchange accounts.
The Public Folder environment at RIT provides folders that can be shared between people in our mail system. This can be a handy feature for those who need to collaborate on work, and is also used for archives for campus mailing lists. However, as you may imagine, the number of messages and the amount of data stored in Public Folders can be quite large.
In order to prevent Apple Mail from checking each Public Folder for unread messages when you connect to your Exchange account, you must disable the "Automatically synchronize changed mailboxes" option. It is found in Mail's preferences for your Exchange account, under the "Advanced" tab.
Turning off this preference is essential for responsive performance from Apple Mail when you connect to your Exchange mailbox. It is also exceedingly helpful for those who are on slower connections, such as RIT's DialIP dial-up service or conference/international links -- but it is still beneficial to anyone who is using the campus network or another ISP.