We discovered that a manual sync (sync_client -u) on the active backend node removes the DELETED folder of the mailbox on passive node.. Means that the delayed delete policy (100 days) for the mailboxes is not working in that case.
Example with logfiles:
Deleted mailbox domain.de!user.test^be2.yellow via Webmail
Logfile active backend node
root@PrimaryServer [0] ~ # grep 'user.test^be2.yellow' /app/appdata/maillog
Aug 25 14:25:14 PrimaryServer cyrus-imapd/imap[3625]: Rename: domain.de!user.test^be2.yellow -> domain.de!DELETED.user.test^be2.yellow.57BEE3A9
Aug 25 14:25:14 PrimaryServer cyrus-imapd/imap[3625]: Deleted mailbox domain.de!user.test^be2.yellow
Aug 25 14:25:14 PrimaryServer cyrus-imapd/sync_client[15759]: UNSUB test^be2@domain.de domain.de!user.test^be2.yellow
Aug 25 14:25:14 PrimaryServer cyrus-imapd/sync_client[15759]: MAILBOX domain.de!user.test^be2.yellow
Aug 25 14:25:14 PrimaryServer cyrus-imapd/sync_client[15759]: MAILBOX domain.de!DELETED.user.test^be2.yellow.57BEE3A9
Logfile passive backend node
root@SecondaryServer [2] kolab-fast # tail -1000 /app/appdata/maillog | grep test.be2
Aug 25 14:24:24 SecondaryServer cyrus-imapd/imaps[16525]: login: localhost [::1] test.be2@domain.de plaintext+TLS User logged in SESSIONID=<cyrus-imapd-16525-1472127852-1-9995344664534961608>
Aug 25 14:24:31 SecondaryServer cyrus-imapd/imaps[16525]: USAGE test.be2@domain.de user: 0.299954 sys: 0.021996
Aug 25 14:25:14 SecondaryServer cyrus-imapd/syncserver[24364]: Rename: domain.de!user.test^be2.yellow -> domain.de!DELETED.user.test^be2.yellow.57BEE3A9
Aug 25 14:25:15 SecondaryServer cyrus-imapd/syncserver[24364]: Deleted mailbox domain.de!user.test^be2.yellow
DELETED folder on active node is existing
root@PrimaryServer [0] ~ # ls -la /app/kolab-fast/spool/domain/m/domain.de/u/DELETED/user/test^be2/
total 48
drwx------ 3 cyrus mail 4096 Aug 25 14:25 .
drwx------ 986 cyrus mail 36864 Aug 25 14:25 ..
drwx------ 3 cyrus mail 4096 Aug 25 14:25 yellow
root@PrimaryServer [0] ~ #
DELETED folder on passive node is existing
root@SecondaryServer [0] kolab-fast # ls -la /app/kolab-fast/spool/domain/m/domain.de/u/DELETED/user/test^be2/
total 48
drwx------ 3 cyrus mail 4096 Aug 25 14:25 .
drwx------ 852 cyrus mail 36864 Aug 25 14:25 ..
drwx------ 3 cyrus mail 4096 Aug 25 14:25 yellow
root@SecondaryServer [0] kolab-fast #
Running a manual sync for user on active backend node
root@PrimaryServer [0] ~ # su -s /bin/bash -c "/usr/lib/cyrus-imapd/sync_client -v -l -u test.be2@domain.de" - cyrus
USER test^be2@domain.de
Logfile passive backend node -> Mailbox domain.de!DELETED.user.test^be2.yellow.57BEE3A9 was deleted
Aug 25 14:25:54 SecondaryServer cyrus-imapd/imaps[24153]: login: localhost [::1] test.be2@domain.de plaintext+TLS User logged in SESSIONID=<cyrus-imapd-24153-1472127946-1-9738847345406888997>
Aug 25 14:26:03 SecondaryServer cyrus-imapd/imaps[24153]: USAGE test.be2@domain.de user: 0.298954 sys: 0.024996
Aug 25 14:28:47 SecondaryServer cyrus-imapd/syncserver[25432]: Deleted mailbox domain.de!DELETED.user.test^be2.yellow.57BEE3A9
DELETED folder on passive backend node is not existing anymore
root@SecondaryServer [0] kolab-fast # ls -la /app/kolab-fast/spool/domain/m/domain.de/u/DELETED/user/test^be2/
ls: cannot access /app/kolab-fast/spool/domain/m/domain.de/u/DELETED/user/test^be2/: No such file or directory
root@SecondaryServer [2] kolab-fast #