I have tried the following:
mailx -s "test" firstname.lastname@example.org
which says bash no command found.
also have tried : Mail -s "test" email@example.com
which says /usr/sbin/sendmail: No such file or directory
even though the file name is valid
For basic sending, just try
Now, as Sleske mentioned, you might need to modify /etc/mail/sendmail.mc to configure this differently. But if you're just wanting to send, and not receive, E-Mails locally, and you've already got an MX record for xyz.com, I think you should be all set.
Also, given your 'command not found' issues, double check you have MTAs installed. use
You first need to set up a local mail server, and configure it to send mails (usually by passing them on to your ISP's relaying mail server).