feedmail-message-id-generator

feedmail-message-id-generator is a variable defined in `feedmail.el'.
Its value is

t

Documentation:
Specifies the creation of a Message-Id: header field.

If nil, nothing is done about Message-Id:.

If t, a Message-Id: header of a predetermined format is produced, but
only if there is not already a Message-Id: in the message. A value of
t is equivalent to using the function feedmail-default-message-id-generator.

If neither nil nor t, it may be a string, a fiddle-plex, or a function
which returns either nil, t, a string, or a fiddle-plex (or, in fact,
another function, but let's not be ridiculous). If a string, it
should be just the contents of the header, not the name of the header
itself nor the trailing newline. If a function, it will be called
with one argument: the possibly-nil name of the file associated with
the message buffer. For an explanation of fiddle-plexes, see the
documentation for the variable `feedmail-fiddle-plex-blurb'. In all
cases the name element of the fiddle-plex is ignored and is hardwired
by feedmail to either "Message-Id" or "Resent-Message-Id".

You should let feedmail generate a Message-Id: for you unless you are sure
that whatever you give your messages to will do it for you (e.g., most
configurations of sendmail). Even if the latter case is true, it
probably won't hurt you to generate your own, and it will then show up
in the saved message if you use Fcc:.

You can customize this variable.