A similar issue was discussed on the Cpanplus-devel list; one suggestion raised there was to install Net::Domain to attempt to get the domainname. You might try installing that, if it's not already present. Another option, as described in Test::Reporter::Mail::Util, is to set the MAILDOMAIN environment variable.

All this assumes though that your domainname is valid (ie, is associated with a real IP address). If you don't have a permanent IP address, your ISP will probably have an SMTP server that you can use for outgoing email; you could try using mail_send_args of Test::Reporter to specify the smtp server to pass to Mail::Mailer.