The Following should be safe for any(rpm/deb) default systems:
chown -R gromox:gromox /var/lib/gromox/user
chmod -R 0770 /var/lib/gromox/user
with that said.. if i look into an actual server with the following command and output...
# find /var/lib/gromox/user/ ! -user gromox -ls
154280 0 drwxrwx--- 1 grommunio gromox 4 Dec 2 02:07 /var/lib/gromox/user/5
2046729 0 drwxrwx--- 1 grommunio gromox 46 Dec 2 02:07 /var/lib/gromox/user/5/3
2046730 0 drwxrwx--- 1 grommunio gromox 56 Jan 18 15:54 /var/lib/gromox/user/5/3/exmdb
2046739 1300 -rw-rw---- 1 grommunio gromox 1331200 Jan 18 15:54 /var/lib/gromox/user/5/3/exmdb/exchange.sqlite3
2046741 76 -rw-rw---- 1 grommunio gromox 77824 Dec 2 02:07 /var/lib/gromox/user/5/3/exmdb/midb.sqlite3
2046731 0 drwxrwx--- 1 grommunio gromox 42 Dec 2 02:07 /var/lib/gromox/user/5/3/tmp
2046732 0 drwxrwx--- 1 grommunio gromox 0 Dec 2 02:07 /var/lib/gromox/user/5/3/tmp/imap.rfc822
2046733 0 drwxrwx--- 1 grommunio gromox 0 Dec 2 02:07 /var/lib/gromox/user/5/3/tmp/faststream
2046734 0 drwxrwx--- 1 grommunio gromox 80 Jan 18 15:54 /var/lib/gromox/user/5/3/eml
2046735 0 drwxrwx--- 1 grommunio gromox 80 Jan 18 15:54 /var/lib/gromox/user/5/3/ext
2046736 0 drwxrwx--- 1 grommunio gromox 1244 Jan 18 15:54 /var/lib/gromox/user/5/3/cid
2046737 0 drwxrwx--- 1 grommunio gromox 44 Dec 5 08:33 /var/lib/gromox/user/5/3/config
2046738 8 -rw-rw---- 1 grommunio gromox 6291 Jan 18 13:04 /var/lib/gromox/user/5/3/config/portrait.jpg
at least for the basic content listed above we see that a relatively new Mailbox has a bit different Ownership which align with the settings from:
# grommunio-admin config get options
...
fileGid: gromox
filePermissions: 0o660
fileUid: grommunio
...
userAcceleratedStorage: null
userPrefix: /var/lib/gromox/user/
userStorageLevels: 2
...
if i'm not mistaken.... o_0.
So basically file*-settings for userPrefix with the basic knowledge that directories have to be set with +x (executable).