If I understand email correctly, I need to setup the smtp configuration before my client can send email to outside domain? If that is the case, where do I set this up in linux's mail/mailx command?
mail/mailx expects to be able to send mail via a
The convention is that there is a command
While this convention originated with the "sendmail" mailserver (duh), most other mail servers provide such a sendmail command. So you need to setup a locally running mail server, at least for sending mail.
There are many to choose from, most Linux distros set one up for you, usually postfix or Exim. Check your distro's docs for details.
If you are on Fedora/Redhat/Centos either Postfix or sendmail is available. Postfix is probably the easier to configure. You will also need an MAA such as Dovecot to retrieve your incoming mail.