[Dovecot] Many messages clustered around the same date.saved value

[Joseph Tam]

Subject: Different user messages clustered around the same date.saved value

After updating dovecot to 2.1.3, I can now use "doveadm expunge -A ..."
to iterate through all user trash folders and expunge old messages.

However, I noticed a strange thing: querying what would have been deleted

 	doveadm -ftab fetch -A "date.saved" mailbox Trash savedbefore 7d

showed many date.saved values are clustered around the same
timestamp, even among different user's Trash mailbox.  One user's trash
mailbox having the same date.saved is explained by a user deleting a
lot of message at one time, but I can't explain why many different users
would have messages with the same (or closeby) date.saved value.

For example, the output of the above query on my system showed the 10s
window /2012-03-05 18:08:0[0-9]/ matched 7658 messages among 22 different
user Trash mailboxes, which is statistically unlikely.

I did't see anything special in the dovecot logs at this time to
explain this.  What would cause this?

Joseph Tam <jtam.home at gmail.com>