The best way to set this up would be:
Adjust your MX record for the domain to point to the Mitea Maildefender VM.
On Mitea, you create domains, and a transport target (which is the IP/host of the target mail server). Once Mitea scans and processes the mail, depending on how your filtering is setup (default is that ONLY non-SPAM gets forwarded), it will be passed along to the mail transport.
Mitea has 3 levels of spam:
Non-SPAM
Possible SPAM
Definite SPAM
By default, only the Non-SPAM (messages with a green light) will be forwarded. However, you can adjust the rules, and even the scoring. These can also be adjusted by recipient (setup users WITHIN a domain, not via the /admin).