Hi Roberto,
There are multiple options of configuring the SMS Appliance.
1) If you are having multiple servers than you can keep it on the same DMZ segment or in a different as there is no relevent with inbound & outbound.
2) For accessing a web console you can define certain machine of admin's for accessing not all the local LAN for security reasons.
3) You can configure Control center & Scanner on the single appliance & you can also receive & send email with same Public IP as there is new new feature of delivery address binding in the current verison of SMS.
Kindly let us know if you need any more clarification on the same.
Regards,
Laeek