My Ubuntu is set to run ntp before dovecot at startup. (See my earlier question: The rc0.d,rc1.d,... directories in /etc)
However, I suspect that dovecot may have finished executing before ntp finished (even if the ntp started executing earlier).
What logs should I look at to find out whether this was the case?
(In case you're interested: The reason for my suspicion is a message like this in /var/log/mail.log when I rebooted my machine (it had been a long time since the last reboot):
dovecot: Fatal: Time just moved backwards by 43 seconds. This might cause a lot of problems, so I'll just kill myself now.
This was fixed by rebooting the machine again, providing further support for my suspicion.)