Actually, it looks like that line is depricated, according to cPanel Forums

There's a line that reads:

hostlist relay_hosts = lsearch;/etc/relayhosts : \
localhost
that's supposed to open the relay for the localhost machine. Also on the above page I see advice that the nobody user should be allowed to send mail (in Tweak Settings). I changed that, but still I can't get ColdFusion to relay mail through localhost.