When the rewriting rules submit an address to the local mailer, the user name is passed through the alias file. If no alias is found (or if the alias points back to the same address), the name (with :maildrop appended) is then used as a key in the user database. If no match occurs (or if the maildrop points at the same address), forwarding is tried.
If the first token of the user name returned by ruleset 0 is an @ sign, the user database lookup is skipped. The intent is that the user database will act as a set of defaults for a cluster (in our case, the Computer Science Division); mail sent to a specific machine should ignore these defaults.
When mail is sent, the name of the sending user is looked up in the database. If that user has a mailname record, the value of that record is used as their outgoing name. For example, I might have a record:
If a maildrop is found for the user, but no corresponding mailname record exists, the record :default:mailname is consulted. If present, this is the name of a host to override the local host. For example, in our case we would set it to CS.Berkeley.EDU. The effect is that anyone known in the database gets their outgoing mail stamped as user@CS.Berkeley.EDU, but people not listed in the database use the local hostname.