The directory channel is similar in function to the alias file. The alias file is only used when the addressee is on the local system, or matches a channel marked with the aliaslocal channel keyword, while the directory channel provides aliasing for other systems or pseudo domains which your system manages. The directory channel also provides facilities for looking up aliases using mechanisms other than a PMDF database.

The directory channel is used to set up pseudo domains --- systems which exist only in a logical sense. A directory channel is used to transform the mailbox names associated with such a pseudo domain into mailboxes on real systems. Such a scheme can be used to standardize naming conventions for groups of disparate systems.

The directory channel includes special handling for subaddresses, akin to that for the local channel; see Section 2.3.4.70 and Section 3.1.1.6.

The transformations applied by the directory channel can be derived from a number of different information sources:

• Databases can be used, with a separate one for each pseudo domain. These databases are in the same format as PMDF's domain and alias databases and are created with the PMDF CRDB (OpenVMS) or pmdf crdb (UNIX or NT) utility.
• LDAP and X.500 look ups can be used, with a separate set of defaults for each pseudo domain.
• A database providing the names of ALL-IN-1 distribution list files can be used. Addresses are then tranformed into the contents of the corresponding ALL-IN-1 distribution list (OpenVMS only).
• CCSO/qi/ph look ups can be used, with a separate set of defaults for each pseudo domain.

Note that with appropriate use of the aliaslocal channel keyword, the alias database can be used to implement functionality similar to the directory channel's crdb database type of lookup on arbitrary pseudo domains. Similarly $(text) rewrite rule substitutions and the PMDF general database⁶ can be used to implement similar functionality. Such alias database or general database use avoids the overhead of additional channel processing incurred by the directory channel. But although such schemes may be more efficient than using a directory channel, the directory channel's crdb lookups do have some additional features such as support of duplicate usernames, support of alternate postmaster return addresses, and better diagnostic messages when illegal addresses are used.