On 11 Sep 2009, at 09:06, Frank Elsner wrote:

Sep 10 21:21:02 seymour dovecot: dovecot: Fatal: Time just moved
backwards by 434 seconds. [ ... ] Sep 10 21:50:55 seymour ntpd[9104]: time reset +434.824810 s Sep 10 21:26:36 seymour ntpd[9104]: no servers reachable

What might happened? And where to ask also?

This is not a dovecot problem. It's a timekeeping problem. dovecote
detected that your computer went back in time and didn't like that.
Check on the NTP mailing lists and web site for more information about
why your time-keeping failed.

It looks like you have a broken NTP setup. Your NTP daemon decided to
change your computer's time by 434 seconds. This "never happens". ntpd
continuously makes lots of small adjustments to the computer's time of
day clock so that time always goes forward as it synchronises that
clock to a more reliable and accurate time source. It usually only
jumps the time by intervals of several seconds or more at boot time
(or if it's run by hand and forced to do that).

BTW, you seem to be running on a broken operating system too. Good
ones don't permit time travel. The only way they allow the time of day
back to be put back is when the system is in a privileged, single-user
state. Messing with the system clock causes security holes -- eg
replay attacks --and breaks application software, notably make which
relies on timestamps. And as you've just found out, it breaks dovecot too.