If you wish email messages to be delivered from the various automatic subsystems on your Mythbuntu machine, you can install sendmail and the mailutils packages (see above) on the system. Then, you can hack sendmail.mc on the Mythbuntu machine to use your regular SMTP server to relay the mail. Begin by adding a SMART_HOST line at the end of the macro file (probably /etc/mail/sendmail.mc) and aim it at your SMTP server:
dnl #
dnl # Smart host (the guy who really delivers the mail)
dnl #
define(`SMART_HOST',`pri-host')dnl
Rebuild the sendmail.cf file:
sudo sendmailconfig
Answer yes to all of the questions. This will restart sendmail at the same time.
You should now be able to send mail to the main mail server from the local Mythbuntu machine (if relaying is denied, you may need to add the Mythbuntu machine's address to relay-domains on the mail server). You also might want to alias the following users in /etc/aliases on the local machine:
root: joeblow@mydomain.com
mythuser: joeblow@mydomain.com
Where "mythuser" is the name of the user that Mythbuntu was installed under (i.e. the initial user from setup) and joeblow@mydomain.com is the name of the user whom the mail should delivered to on the smart host.
As long as the aliased name contains a domain name, it will be forwarded by the local sendmail to the SMART_HOST. If the domain name is marked as a local domain on the main mail server, the mail will get delivered there. Note that you need to rebuild the aliases database and then recycle sendmail on the Mythbuntu machine when you add aliases to /etc/aliases:
sudo newaliases
/etc/init.d/sendmail restart