300-720: Securing Email with Cisco Email Security Appliance

SPF authorizes the IP address from which a receiving mail server observes SMTP mail. A mail host using private address 10.1.10.23 is translated to public address 199.209.31.21 when it sends Internet mail, so the SPF policy for mycompany.com must authorize 199.209.31.21 with the ip4 mechanism. SPF evaluation tests the client host emitting mail, and ip4 matches the specified IPv4 address.