is OK if i set all my email virtual account (user that
doesn't have system account just for email only) UID and
GID same as UID and GID of courier ?. I use user courier
instead of daemon to run several process for the courier
There is nothing that will inherently prevent such a thing.
Of course, you should understand fully well the implications of such
configuration. Particularly, if mail filtering is enabled and individual
recipients have the ability to install an arbitrary mail filter, a hostile
user will be able to completely take over the entire mail system.