Apple's support for Microsoft Exchange 2007 in Snow Leopard has been a welcome addition to the Mac OS, especially in corporate environments. Some people implementing Exchange in Mail have been running into a problem where the connection to the server will either hang or go offline.
This problem seems to happen over time, where upon restarting the computer the system will maintain a connection for an hour or so and then start disconnecting. Some people have had to remove and recreate their exchange accounts for the problem to go away, only to have it pop up again, and others have had success in waiting in offline mode for an hour or so before the connection will re-establish.
The problem may have to do with the routing or name resolution of the mail server in the corporate network, which has been overcome by people who've added their mail server to the system's "hosts" file, which serves the same purpose as a DNS server. To do this, open the terminal and enter the following command:
sudo pico /etc/hosts
Authenticate (your password will not show) and you should see the hosts file open in a terminal text editor. Press the down arrow to get to the bottom of the file, and enter your mail server's ip address, followed by the tab key and then enter the server's domain name.
Following this, press control-O and confirm overwriting the file, and then press control-X to quit the editor. If you do not know your mail server's IP address, you can find it by entering the following command in the terminal: