SpamFilter is not a proxy, as when it receives an email that is deemed "clean" it will modify it by adding some headers in it, and then will connect to your destination SMTP server(s) to deliver it. It will then wait before accepting the email from the sender until your SMTP server has fully accepted it, so as to practically eliminate all risk of backscatter. When doing this, SpamFilter is behaving as an actual email client, which requires an established TCP session between your destination SMTP server and the SpamFilter server. In the TCP session your SMTP server must see SpamFilter's true IP, otherwise the SMTP session will never be stablished.

In there it will indicate the IP address of the sender's server that made the connection to SpamFilter. In addition, it will add the following "X-SF-Originating-IP " header with the same IP: