I changed MailFrom="" in /usr/share/logwatch/default.conf/logwatch.conf
That altered "From:" in the headers, but "Sender:" is still "email@example.com"
EC2 Linux AMI beta, Postfix is the mailer
Arrived here via google with the same problem.
Adding the "-f user" option to sendmail in
Turns out logwatch.pl processes
Edit or remove the