=head2 previoushop

Adds the previous host through which a mail message passed to the Path
header of the message.  It does this by looking through the raw Received
headers, finding the first Received header that specifies a host from
which the message was received, and prepending it to the Path header (or
creating a new Path header if none already exists).

If no Received headers that contain a "from" clause are found (which may
be the case for mail originating locally), nothing is done.

The reason for this module is to support news to mail and mail to news
gateways for the same unmoderated newsgroup.  If the mail to news address
is subscribed to a mailing list and posts to the newsgroup are sent back
to the mailing list, a loop will be created unless something is done to
stop it.  One method of stopping such loops is to alias the hostname of
the mailing list machine out of the feed to the news to mail gateway in
the news server and then use this module to put the hostname of the
mailing list machine in the Path of all articles posted through the mail
to news gateway.

This module takes no arguments or configuration directives and cannot
fail.