feedmail-sender-line

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

nil

Documentation:
If non-nil and the email has no Sender: header, use this value.
May be nil, in which case nothing in particular is done with respect
to Sender: lines. By design, will not replace an existing Sender:
line, but you can achieve that with a fiddle-plex 'replace action.
NB: it makes no sense to use the value t since there is no sensible
default for Sender:.

If not nil, 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 "Sender" or "Resent-Sender".

You can probably leave this nil, but if you feel like using it, a good
value would be a string of a fully-qualified domain name form of your
address. For example, "bill@example.net (WJCarpenter)". The Sender:
header is fiddled after the From: header is fiddled.

You can customize this variable.