feedmail-x-mailer-line

feedmail-x-mailer-line is a variable defined in `feedmail.el'.
Its value is

t

Documentation:
Control the form of an X-Mailer: header in an outgoing message.
Moderately useful for debugging, keeping track of your correspondents'
mailer preferences, or just wearing your MUA on your sleeve. You
should probably know that some people are fairly emotional about the
presence of X-Mailer: lines in email.

If nil, nothing is done about X-Mailer:.

If t, an X-Mailer: header of a predetermined format is produced,
combining its efforts with any existing X-Mailer: header. If you want
to take the default construct and just add a little blob of your own
at the end, define the variable feedmail-x-mailer-line-user-appendage
as that blob string. A value of t is equivalent to using the function
feedmail-default-x-mailer-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 no arguments. 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 "X-Mailer" or "X-Resent-Mailer".

You can customize this variable.