=head2 mailpath

Generates an X-Mail-Path header from the Received headers of a mail
message.  It attempts to figure out the real name of each host that the
mail passed through based on the standard comment syntax used by most
mailers, but this problem is inherently impossible to solve completely.

X-Mail-Path is set to a !-separated list of hosts that the mail has passed
through.  If from the Received headers the name of a host appears to be
trusted, it's given; otherwise the name followed by the IP address in
brackets is given.  If the IP address isn't available, "[UNTRUSTED]" is
appended.

The envelope sender is added to the end of the X-Mail-Path if known; if
not, "UNKNOWN" is added instead.

mailpath takes no arguments or configuration directives and cannot fail.
