FYI,
Create new virtual server, enable mail, add email accounts.
Then - dissable email in server options.
Setup MX outside.
If the local virtual web server sends email to virtual accounts - mail delivered localy.
/etc/postfix/virtual.db still have route to dissabled local virtual email clients.
You must delete those lines and run - postmap /etc/postfix/virtual to update db.
P.S.
On deb etch 4.