MASQUERADE_AS(`host.domain')Masquerading is a very important part of a client/server mail hub configuration. Any machine configured like this will rewrite the SMTP From: header of all outbound mail to look like the message came from the address you're masquerading as. This address must be an address record in DNS, not simply a CNAME, or the remote end will canonicalize the address back to the original name.
The FEATURE(allmasquerade) macro is also important. It will cause the recipient addresses to be masquerades as well as the sender's address. This is often useful on a cluster of machines which masquerade as a central mail server when you would like all addresses on the cluster to look to the outside world like they come from the mail server.