Sam,
Under Mail Policies/HAT Overivew, make sure to select the "inbound" listener. Look at the various Mail Flow Policies, and see if one is appropriate.
If so, you can add this sender's domain, or IPs to the appropriate Sender Group on left. Make sure you check all of the settings in the policy. ex. Whitelist is probably set to allow a lot of mail, but there are other things turned off that you may still want applied to this mail.
If not, you can create a new policy under Mail Policies/Mail Flow Policies, and then a new Sender Group under Mail Policies/HAT Overivew, and tie in the new policy there.
Keep in mind that Sender Groups are really based on IP, and that the sender group table is processed from top to bottom, so the order in the HAT matters.
HTH,
Ken