Today we ran into a dovecot/imap crash on a Xenial box. The Dovecot in question was the patched dovecot-2.2.22.
debug dovecot crash roundcube
At times, Dovecot started spewing messages into dovecot.log about a corrupted index cache file because of “Broken MIME parts”. This happened on Ubuntu/Xenial with dovecot_2.2.22-1ubuntu2.2:
patch xenial dovecot ubuntu