/etc/postfix/main.cf

virtual_mailbox_domains = <domain_name>
virtual_mailbox_base = /var/spool/virtual
virtual_mailbox_maps = hash:/etc/postfix/vmailbox
virtual_minimum_uid = 5000
virtual_uid_maps = static:5000
virtual_gid_maps = static:5000
virtual_alias_maps = hash:/etc/postfix/virtual

/etc/postfix/virtual MailDir

user@address   domain/user/Maildir/

/etc/postfix/virtual MailBox

user@address   domain/user