A list if IP addresses that are trusted by the FortiGate is created. Any email traffic coming in from these IP address will be considered to be non-spammers.

If the FortiGate unit sits behind a company’s Mail Transfer Units, it may be unnecessary to check email IP addresses because they are internal and trusted. The only IP addresses that need to be checked are those from outside of the company. In some cases, external IP addresses may be added to the list if it is known that they are not sources of spam.