The splitmail program, used by metasend and mailto, expects sendmail to be located in /usr/lib/sendmail, not /usr/sbin/sendmail. Thsi works with out-of-the-box RedHat, but may not if changes are done to the MTA.
Fixed in metamail-2.7-21, will be in next release. There were actually quite a few instances of /usr/lib/sendmail throughout metamail, but I think I got them all.