mailhandlers like mh (used in emacs for instance), and exmh,
use sendmail.  In the default configuration of sendmail,
/etc/sendmail.cf should read the database corresponding to
/etc/mail/access and allow relaying of mail from localhost.
/etc/sendmail.cf is not properly configured, so that the
entries in /etc/mail/access are ignored and it is impossible
to send mail using the above mailhandlers.

P.S. reverting to RedHat 5.2's /etc/sendmail.cf and
regenerating ip_allow, name_allow, relay_allow in /etc/mail
is a workaround, but it is perhaps not as secure.

P.P.S. I note that bug 2400 reports the same symptoms.  The
working sendmail.cf is from the rpm 8.8.7-20, the sendmail
rpm in the redhat 6.0 distribution is 8.9.3-10.