The IMTA provides a facility to support mailbox names associated with the local system that do not necessarily correspond to actual users: aliases. Aliases are useful for constructing mailing lists, forwarding mail, and synonyms for user names. A second set of related facilities provides support for "centralized naming," whereby you establish, for instance, mail addresses of the form first.last@acme.com for all of your users. There are several advantages to such centralized naming systems: the addresses are simple, they provide added security in that they make no reference to internal account or system names, and, because they lack reference to account and system names, they are more stable.
Each time an address that matches the local channel is encountered by the IMTA's message submission logic, the mailbox (for example, username) specified in the address is compared against each entry in the alias database or alias file. If a match occurs the alias address is replaced by the translation value or values specified by the alias. An alias can translate into any combination and number of additional aliases or real addresses. The real addresses need not themselves be associated with the local channel and thus aliases can be used to forward mail to remote systems.
Aliases only apply to addresses mapped to the local channel; furthermore, note that since the only addresses truly considered to match a channel are Envelope To: addresses, aliases can only apply to Envelope To: addresses. The IMTA performs alias translation and expansion only after address parsing is completed. The translation values produced by an alias are treated as completely new addresses and are reprocessed from scratch.