I had this problem several days ago (check back through the last week of postings)
I could find no answer in the logs, other than all mail (incoming & outgoing, local and internet) was building up in the queue.
Eventually I simply logged on, did 'shutdown now -r' and when my server came back up, all email was distributed normally. Still don't know why it failed, but all mail seemed delivered Ok, although several of them to my work email account arrived almost 48 hours after the re-boot,
Regards
Craig