You need to create a symbolic link
ln -s /var/qmail/bin/sendmail /usr/sbin/sendmail
or
ln -s /var/qmail/bin/sendmail /usr/bin/sendmail
Depending on the output of mutt -v | grep -i sendmail
The alternative is to configure mutt to use the SMTP protocol to transfer the messages.
http://wiki.mutt.org/index.cgi?MuttFaq