Setup :
Virtual mailboxes with postfix + courier. MDA = maildrop.
maildrop is putting the emails of all the users into /var/vmail/Maildir instead of /var/vmail/domain.tld/email/
file structure :
/var/vmail/domain.tld/user_name/
/var/vmail/domain.tld/other_user_name/
postfix's master.cf ...