After a Scanner filters inbound email, the MTA relays filtered email to a mail server for delivery to recipients. When you configure email delivery for an inbound-only or combined inbound and outbound Scanner, you designate the default local and non-local hosts where filtered inbound mail should be routed.
You can designate an unlimited number of local or non-local mail servers for delivery of inbound mail. Multiple downstream mail servers can improve load distribution and fault tolerance for filtered inbound mail if you accept mail that is addressed to different domains. You must specify the order in which a default relay (static route) delivers inbound email by assigning a preference number to each mail host. The Scanner attempts to deliver email to lower- numbered mail hosts first. If only one mail server is configured, its preference defaults to 1. If you want to load balance, specify equal preference numbers for each server.
Additionally, you can assign a mail server to each domain for which the Scanner accepts inbound email. Each mail server can host multiple local domains.