Postfix + Dovecot + SASL: first systemusers then virtual

[ShinnRa]

Hi, i'm having problems with postfix and dovecot setup. I want it to work like this:

1. postfix/dovecot checks if a specific system user exsts
2. ok: systemuser exists - mail is delivered to ~/Maildir
3. fail: systemuser doesn't exists - check if it's a virtual user
4. ok: virtualuser exists - mail is delivered to /home/vmail/<domain>/<user> (format: Maildir)
5. fail: it's not a systemuser or virtualuser - return 550 error


And i don't know how exacly configure postfix and dovecot as a LDA... I think further configuration to use Amavis and Spamassassin shouldn't hard but now i'm stuck on postfix and dovecot.