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.
It appears that a replacement ssl-proxy-openssl.c from http://www.dovecot.org/list/dovecot/2006-August/015160.html actually helps (and maye really solves the issue; at least with dovecot-1.0-0.rc5).