Ever since I've migraded from courier pop3 to dovecot pop3 I'm constantly getting complains from users that went over system quota for short while and then their pop3 client downloads all messages again. I never happened with courier pop3. I think that dovecot does not handle dovecot-uidlist file safely so it can withstand such situations. rsync and other similar programs always writes to a copy of the file like dovecot-uidlis.xcb453 and on success unlinks old file and renames new one. Dovecot should not assume that write operation on dovecot-uidlist will succeed. Yes, I know I can put metadata like dovecot-uidlist somewhere else, on another partition but that really splits mailbox into multiple locations making for example backups way harder to make, restore, share etc.