feedmail-fill-to-cc

feedmail-fill-to-cc is a variable defined in `feedmail.el'.
Its value is

t

Documentation:
If non-nil do smart filling of addressee header lines.
Smart filling means breaking long lines at appropriate points and
making continuation lines. Despite the function name, it includes
To:, Cc:, Bcc: (and their Resent-* forms), as well as From: and
Reply-To: (though they seldom need it). If nil, the lines are left
as-is. The filling is done after mail address alias expansion.

You can customize this variable.