Each time an address that matches the local channel¹ or any channel marked with the aliaslocal keyword is encountered by PMDF's message submission logic, the mailbox (e.g., 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. If the translation value of an alias is a file name preceded by a <, then the contents of that file are used as a mailing list (e.g., distribution list) and the message is sent to each recipient listed in the file; if the translation value of an alias is an LDAP URL preceded by a < that returns one or multiple addresses, then the message is sent to each address returned. This process is occasionally referred to as "mail exploding". See Section 4.1 for directions on how to set up a mailing list.

Aliases only apply to addresses mapped to the local channel¹ or to channels marked with the aliaslocal keyword; 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. PMDF 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.²

Aliases as well as mailing lists, a special case of an alias, may be tested with the command PMDF TEST/REWRITE/CHECK_EXPANSIONS (OpenVMS) or pmdf test -rewrite -check_expansions (UNIX or NT). See Chapter 31 and Chapter 32 for details. In regards to mailing lists, see the final part of Section 4.1.3.