After upgrading from dovecot-core 2.2.13-12~deb8u4 (jessie) to 2.2.27-3+deb9u2 (stretch), we are seeing this issue appearing again on a few of our customer mailboxes with lots of entries inside mail.log. It seems that in our case especially large mailboxes containing older emails dating back a few years are affected.
We already tried to rebuild dovecot mailbox cache by stopping the dovecot service and deleting the dovecot.index.cache without success: find /var/mail -name 'dovecot.index.cache' -exec rm {} \; Any sugesstions on how to debug this issue? Regards Dominic