As was shown in the previous chapter, the
sendmail program is able to obtain its list of
recipients from the aliases file. It can also
obtain lists of recipients from external files. In this chapter we
will examine the two forms that those external files take: the
:include: form (accessed from the
aliases file) and the individual
file. Because the chief use of the
:include: form of alias is to create
mailing lists, we will first discuss mailing
lists in general, then their creation and management, and then the
user’s ~/.forward file.
A mailing list is the name of a single recipient that, when expanded by sendmail aliasing, becomes a list of many recipients. Mailing lists can be internal (in which all recipients are listed in the aliases file), external (in which all recipients are listed in external files), or a combination of the two. The list of recipients that forms a mailing list can include users, programs, and files.
 RFC defines a mailing list as a pseudo-user’s address that expands to multiple real email addresses. As you will see when we cover the ~/.forward file, real email addresses also can expand to mailing lists.