OSX Mail is a typical Apple product, it has some great features that make it fantastic but it is let down by the tiny details. For example I've been battling with it's 'Synchronize IMAP Account' feature and its ability to almost crash my Scalix mail server. The feature is useful because it creates a local copy of an on-line IMAP account for SpotLight searching and general offline use. Unfortunately if this process is run after a long period of time offline it can cause the Scalix IMAP server to trip up and not allow further IMAP connections.
I've been on the Scalix forums trying to resolve this issue and it seems like the root cause is a hard-coded limit of 17 simultaneous IMAP connections within Scalix. Unfortunately OSX Mail creates multiple simultaneous connections when performing the synchronization process and just to make matters worse it takes its own sweet time in closing them. Consequently it does not take long for the 16 process limit to be reached, the Scalix IMAP server to trip over and the Mail client to be left out in the cold.
The most annoying thing about the problem is that unlike Thunderbird, OSX Mail offers no way to limit the number of simultaneous connections, thus getting around the issue. Fortunately it sounds like the server team will resolve this hard coded process limit bug, perhaps in time for Scalix 11, so the annoying issue maybe resolved once and for all.