This feature allows mail originating from a machine to appear as mail originating from the domain it is in. This feature is intended to allow multiple machines to coexist inside a single domain. In particular it allows machines inside a domain to originate mail to machines outside the domain and make the mail appear as if it is from the domain itself without forwarding outgoing mail through a central router.
For example, domain pootle.com has a mail exchange (MX) record in DNS to the central mail router machine pootle.pootle.com and all the machines within the pootle.com domain generate mail with masquerading set to pootle.com. All pootle user addresses are of the form user@pootle.com. Masquerading makes all outgoing messages appear as if they originated from the domain pootle.com with individual addresses appearing as user@pootle.com. This allows replies to work correctly and provide a uniform view of the pootle.com domain to the email community.
Full details of how to enable masquerading are given in ``Managing masquerading''. The actual implementation of the feature is to rewrite the ``sender'' and ``From'' fields of each new message to have the domain specified in the ``Mail Comes From'' attribute. Messages that already have a ``from'' domain are not rewritten (so messages passing through the machine are left alone).