An excellent way of protecting your MS Exchange or Lotus Notes servers is to place a MailScanner server ‘in front’ of them. To do this with Postfix and MailScanner:
1. Edit main.cf by editing or amending the following:
relay_domains = <your domain>, <your domain2> etc
transport_maps = hash(Or the db type your OS uses):/etc/postfix/transport
2. Edit or create /etc/postfix/transport and add the following:
<your domain> smtp:[ip.address.of.exchange]
<your domain2> smtp:[ip.of.lotus.server]
3. Execute the following:
postmap transport
4. Pass test messages through the MailScanner gateway to ensure mail is routed correctly.
5. Block all external SMTP access to the Groupware box that you are protecting by
a) Edit DNS MX records to suit the new MailScanner server b) Firewall port 25 from external connections to your Groupware box
Please note now you have a mail gateway it is recommended that you read here