I think I found the solution, which I provide here for others:
Recall I said I'm using PHP5-cgi. After some more Googling I made the following addition in /etc/e-smith/templates-custom/etc/php5/php.ini/80ModuleSettings02mailfunction
[mail function]
SMTP = localhost
sendmail_from = me@localhost
sendmail_path = usr/sbin/sendmail -t -i
The sendmail_path line is what I added.
Of course one must also do
# expand-template /etc/php5/php.ini
# service httpd-e-smith restart
Built-in PHP mailer function tested working in TikiWiki (in /opt/wiki) Not tested with Mediawiki in /opt as I gave up on that awhile ago, but likely will also succeed.