Description of problem:

Many people observe hangs in Thunderbird during attempts to store
archival copies of an outgoing mail.  The problem is somewhat elusive
(it seems to depend on a timing, layout and other external factors
and not everybody is hit by it) and possibly really a Thunderbird issue
but this particular dovecot version triggers it.  Version 1.5.0.5
of Thunderbird does not really solves that regardless of some
reports to the contrary.

See a thread with http://www.dovecot.org/list/dovecot/2006-August/015138.html
for more insight.  This thread actually starts with
http://www.dovecot.org/list/dovecot/2006-July/014693.html

How reproducible:
Depends on your "luck".  Some accounts seems to be hit regularly,
other do not run into that at all.