This problem also happened when I first upgraded to the Media Temple DV 3.0 server. Apple Mail just stopped receiving emails through my IMAP email accounts and showed a little error icon by the account. All emails were showing up fine in web mail.
It turns out that Apple Mail, and apparently Thunderbird, use lots of simultaneous connections with the server to improve performance. Unfortunately the DV server by default can't handle this number of connections.
Media Temple have written an excellent article on the issue but it's techy title meant I didn't find it for a while! Hope this brief article helped you find it.






