A mailing list address is a special address created through the alias file or alias database; see Chapter 3 for general background on aliases, the alias file, and alias database. Associated with each mailing list address is a text file which contains one or more mail addresses, or an LDAP URL that returns one or more mail addresses. Note that an LDAP query URL can return multiple addresses either because the LDAP query matches multiple entries containing a desired attribute(s), or because the LDAP query matches a multivalued attribute of a single entry.) This text file or LDAP URL is sometimes referred to as the mailing list or distribution list. When a message is received by PMDF for the mailing list address, the message is then passed on to each address specified in the mailing list file or LDAP URL. Note that addresses in that file or addresses returned by the LDAP URL can themselves be aliases or mailing list addresses.

A mailing list address alias with associated mailing list file file-spec is specified in the alias file with an entry of the form

alias: <file-spec, named-parameters, error-return-address, \ reply-to-address, errors-to-address, \ warnings-to-address, comments

A mailing list address alias with associated LDAP URL ldap-url is specified in the alias file with an entry of the form

alias: <ldap-url, named-parameters, error-return-address, \ reply-to-address, errors-to-address, \ warnings-to-address, comments

Similar definitions can also be made in the alias database, (though of course omitting the colon, as just white space separates the alias from its definition in the alias database).

The parameters following the file specification, file-spec, or LDAP URL, ldap-url, are optional.

file-spec must be a full file path specification (device, directory, etc.). All files included in this fashion should, like the alias file itself, be world readable.¹ Addresses should appear one per line in this file and be in RFC 822 format; the addresses can either be "real" addresses or further aliases (but not of the form "alias: <file-spec"). Mailing list files can include comment lines as well as references to include files via the include operator, <.

ldap-url must be a standard LDAP URL, with the host and port omitted; (host and port are instead specified via the LDAP_HOST and LDAP_PORT PMDF options; see Section 7.3.2. That is,

ldap:///dn[?attributes[?scope[?filter]]]

where dn is required and is a distinguished name specifying the search base. The optional attributes, scope, and filter portions of the URL further refine what information to return. For a mailing list, the desired attributes to specify returning would typically be the mail attribute (or some similar attribute). The scope can be any of base (the default), one, or sub. And the desired filter might be to request the return of all objects that are in Department X or that have, say, the "member-of-list-y" attribute. Certain substitution sequences can be used to construct the LDAP search URL; see Table 3-1 for details.

The action of parameters that can add headers can be modified by the special characters shown in Table 4-1, by appending the special character at the end of the value for the parameter.

Table 4-1 Parameter Action Modifiers

Character	Description
	Insert if not already present; inserts as a Resent- if already present
*	Only insert if not already present
&	Insert if not already present; add to old field if already present
^	Delete any old field present; always insert the new field
\	Delete old field and don't insert a new one