If you're constantly getting messages from Mac Mail that the mail server could not be reached, it may have something to do with the frequency of which it is checking for new mail. If someone sat next to you all day and constantly tapped you on the shoulder to ask if there was any new mail, you'd get pretty annoyed... this is kind of what can happen between Mac Mail and our mail server.

With the IDLE function enabled, Mac Mail will make a continuous connection to the server, and this will contribute to the problem. Instead, you can configure Mac Mail to periodically check for new mail. A reliable frequency for the idle feature is every 5 minutes to ensure that Mac Mail is cleaning out any lingering processes.

To disable the IDLE feature:
  1. Select Mail | Preferences from the menu.
  2. Choose Accounts.
  3. Select the desired IMAP account.
  4. Go to the Advanced tab.
  5. Uncheck Use IDLE command.