Downloading emails via IMAP repacks the mailbox and resets the seen flag

[Timotheus Pokorra]

Hello,

I am using Cyrus IMAP 2.5 with Kolab 3.4 on CentOS7.
But I think this problem is not related to Kolab, so I am posting on
this mailing list.

I have a reproducable problem when connecting to a mailbox from
Thunderbird through IMAP for the first time, that some messages get
expunged and all emails are set to unread.
If I run reconstruct again on the mailbox, the expunged emails are restored.

in the log file I see:
Expunged 63 messages from mydomain.de!user.myuser
Repacking mailbox mydomain.de!user.myuser version 10

I have no idea what this means. Any ideas?

I am also wondering if the seen state is still stored in
/var/lib/imap/user/... myuser.seen, or actually in the cyrus.index
file in /var/spool/imap?
https://cyrusimap.org/mediawiki/index.php/Cyrus_2.4_Changes says:
"User seen flag moved into cyrus.index, the most common case"
but the myuser.seen file still gets updated, according to the file
modification time

Thank you for your help,
  Timotheus