PMX in AWS

Hi,

Apologies for the noob question. i just want to seek advise from anyone on how i will proceed with my plan. our company has an existing email server hosted in AWS, i want to augment the mail traffic making it secure by installing an Sophos PMX appliance in AWS. how do i route the incoming and outgoing traffic to Sophos PMX?

Incoming - what do i need? do i need to assign an Public IP for the PMX? do i need to change the existing MX record pointing to Sophos PMX Public IP? 

Outgoing - i understand i need to configure relayhost or smarthost from the email server pointing the the ip address of the PMX to relay outgoing traffic. what else do i need?

Thanks in advance for any input.