Ensure that you have properly set up the path to Sendmail OR the SMTP server in the rlconfig.pm. Whichever one you are not using, comment it out using a hash mark (#). See example below:

Beginning on line 34, find these lines:

# Path to sendmail program
# (If installed on a Windows server, you have to include the file
# extension .exe, e.g. $rlmain::sendmail = 'd:/scripts/sendmail.exe';)
$rlmain::sendmail = '/usr/sbin/sendmail';

# SMTP-server
# If the $rlmain::sendmail variable is disabled, e.g. through a #
# character before it, Ringlink will try to send messages directly
# from Perl instead. In that case, the variable below shall be
# assigned the host name of your SMTP-server.
#$rlmain::smtpserver = 'SMTPSERVER';

In this example, the SMTP server is not being used. Note the # mark before $rlmain.

If you try SMTP first and do not get any mail, try Sendmail or vice versa. Many hosts do not allow SMTP, so check with you web host's FAQ.