feedmail-from-line

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

t

Documentation:
If non-nil and the email has no From: header, use this value.
May be t, in which case a default is computed (and you probably won't
be happy with it). May be nil, in which case nothing in particular is
done with respect to From: lines. By design, will not replace an
existing From: line, but you can achieve that with a fiddle-plex 'replace
action.

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 "From" or "Resent-From".

A good value would be a string fully-qualified domain name form of
your address. For example, "bill@example.net (WJCarpenter)". The
default value of this variable uses the standard elisp variable
`user-mail-address' which should be set on every system but has a decent
chance of being wrong. It also honors `mail-from-style'. Better to set
this variable explicitly to the string you want or find some other way
to arrange for the message to get a From: line.

You can customize this variable.