What mail server are you using? This is usually done through the mail server program. Within MailEnable you can open the ME Admin MMC->Servers->Localhost->Connectors->SMTP Properties->Inbound TAB->Port Settings [Button]->"Listen on alternate port"
By default you can only configure two ports which by the sounds of things will be enough for you but if you wanted to setup more (maybe for SSL) then this can be done.