What I mentioned here:
Another clue appears to be that occasionally some users accessing the mail server from Outlook using POP3 get authenticated against OD (which fails) instead of AD. I'm not sure what's triggering that. Restarting the mail service does not solve the issue, though restarting the server does.
...appears to be incorrect. I just had an AD password expire on an account, and the log indicated OD. I suspect that is by design based. The issue I had previously observed with that users account failing to authenticate must be unrelated. Still see plenty of the POP3 timeouts if anyone has any feedback on that...