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As part of the MTA's central role of routing messages, the MTA must recognize and potentially transform addresses to determine whither to route them. Aliases -- addresses that translate to one or more other addresses -- are a fundamental part of the processing. The MTA supports various mechanisms for implementing aliases, and many variants on alias handling.

Aliases are used for "simple" forwarding (which may not be all that simple), for supporting "hosted domains", for defining "mail forwarders" (mail groups), for defining true mailing lists (which have additional semantics beyond mail groups), and for "centralized naming". The term address reversal is used for the process (and techniques) of esthetic transformations of addresses (as in the appearance of addresses in header lines) for purposes of centralized naming, or conforming to site addressing conventions.

Typical site provisioning nowadays is to store the bulk of user definitions (and therefore local domain definitions), as well as group and mailing list definitions, in an LDAP directory. The MTA supports such usage, often referred to as Direct LDAP configuration.

The MTA also supports the older style of defining user aliases via the MTA's alias file (legacy configuration), or alias options (Unified Configuration) -- techniques which are still used for special users (e.g., postmaster) or special, limited purposes even in primarily LDAP-based configurations.

Note that the MTA limits aliases, and addresses in general, to a maximum of 256 characters. (Internet mailers were originally only required to support domain names up to a length of 63 characters, though support for lengths up to 255 characters was recommended, and similarly are only required to support a local-part -- that is, the part to the left of the "@" sign -- of up to 64 characters. See for instance RFC 1123, Requirements for Internet Hosts, Section 2.1, and RFC 5321, SMTP, Section Keep such limits in mind if you wish your addresses to work reliably on the Internet.) This limit of 256 characters is on the actual address itself; the RFC 822 "phrase" more commonly referred to as a personal name is a separate string with its own 256 character limit.

See also: