Dedicate Spam Box Outside Iredmail Srv how

have purchased latest Iredmail Pro.
Presently we have a setup where Antispam Box is Kept outside Mail Server and it is acting as MX of our doman xyz.com.I want to implement same setup using iredmail where incoming mails should be routed through Spam Box but out going mails will bypass(no filter) the spam box.What changes should i make?
whether the changes should be  on Iredmail end OR Spam box end?