In our office we were having troubles. Trouble with email, which is no good in an office. I thought i’d put our solution up here so someone might find it with Google some day.
The symptoms were numerous: We were unable to reply to certain addresses, couldn’t add attachments, etc.
The solution we finally got from Apple was this:
1. Shut down Mail.app
2. In System Preferences go to Network.
3. In the Network box choose “Network Port Configurations”
4. Uncheck all boxes
5. Apply Now
7. Go back, recheck the connections you need to get online.