Also is there a place to specify the addresses that are coming from DYNDNS.
{host definition for the IP of DynDNS service} -> SMTP-Alt -> External (WAN) Address
Traffic Source = {host definition with IP of the DynDNS service}
Traffic Service = SMTP-Alt
Traffic Destination = External (WAN) (Address)
NAT mode = (DNAT) Destination
Destination = {host definition for mailserver}
Destination Service = {Empty or SMTP, depending on your server's configuration}
Traffic Source = {host definition with IP of the DynDNS service}
Traffic Service = SMTP-Alt
Traffic Destination = External (WAN) (Address)
NAT mode = (DNAT) Destination
Destination = {Empty!}
Destination Service = SMTP
Traffic Source = External (Address)
Traffic Service = SMTP
Traffic Destination = {host definition with IP of the DynDNS service}
NAT mode = (SNAT) Source
Source = {Empty!}
Source Service = SMTP-Alt