feedmail-nuke-empty-headers

feedmail-nuke-empty-headers is a variable defined in `feedmail.el'.
Its value is

t

Documentation:
If non-nil, remove header lines which have no contents.
A completely empty Subject: header is always removed, regardless of
the setting of this variable. The only time you would want them left
in would be if you used some headers whose presence indicated
something rather than their contents. This is rare in Internet email
but common in some proprietary systems.

You can customize this variable.