I think the requested minor compatibility fix for the -f option cannot cause harm for other applications, therefore I fixed it upstream:
http://msmtp.git.sourceforge.net/git/gitweb.cgi?p=msmtp/msmtp;a=commit;h=72dd9a2018df7ed5fd81375056db79544c4d783c -- To UNSUBSCRIBE, email to [email protected] with a subject of "unsubscribe". Trouble? Contact [email protected]

