I guess we have to make sure user/group "vmail/vmail" and "postfix/postfix" have the same UID and GID.
Since it was reset to different user/group, please check UID and GID of below user/group:
- On primary server: user/group -> admroot
- On secondary server: user -> iredapd, group -> policyd
You can check them with command "id":
# id admroot
# id iredapd
# id policyd
If this is the issue, please also update file owner of /var/vmail.