unrouteable mail domain

[Spencer]

I just installed ModernBill as well as some forums and when users sign up, I get the following e-mail:

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:

xxx@xxx.com
unrouteable mail domain "xxx.com"

Any ideas?

(This is a cPanel server running on RH 9.)

[Spencer]

I always post too early, because I fixed it. :D

Under Tweak Settings: Prevent the user 'nobody' from sending out mail to remote addresses (php and cgi scripts generally run as nobody if you are not using phpsuexec and suexec respectively.)Don't select that.