Chapter 15 Security Profiles : Email filter : Email filter techniques : Return email DNS check
  
Return email DNS check
The FortiGate unit performs a DNS lookup on the If no such record exists, the message is treated as spam.
When you enable return email DNS checking, your FortiGate unit will take the domain in the reply-to email address and reply-to domain and check the DNS servers to see if there is an A or MX record for the domain. If the domain does not exist, your FortiGate unit will treat the message as spam.