When using a Firewall with NAT (Network Address Translation) configure the Firewall in such a way that port 25 (SMTP port) on the outside translates to an IP-number on your destination server.
If you want to point to a different port on the inside use the PAT (Port Address Translation) possibilities of your Firewall

In this way its possible to configure Pinjo to listen to port 26.
This is exceptionally useful when you are running Pinjo on the same machine with only 1 IP-number. Your original mailserver doesn't need to change it's configuration (listen to port 25).

Example

• Install Pinjo on your server and set the listening port to port 26.
  (Mail tab, Incoming mail)
• Configure Pinjo to your destination mailserver on the Mail tab, Outgoing mail. The destination port should be 25 in this case
• Configure your Firewall to translate the outside address to the address where your mailserver and Pinjo are running on (This probably is already done if you were using a mailserver already)
• Configure your Firewall to translate the outside port (port 25 SMTP) to the port where Pinjo is running on (port 26)
  

All mail will be delivered to your Pinjo configuration now, and after checking your mail for spam, Pinjo will forward it to your original mailserver.
Your inside clients dont need to change anything since they are still using your original mailserver to send mail.