mailx will use a local MTA (usually sendmail) to transfer emails. If the machine you're using doesn't have sendmail,
mailx will fail to send the email.
This can be resolved by specifying a mail server directly, in the case below, the SMTP server:
mail -s "subject" -S smtp=smtp://smtp.home.steamr.com "firstname.lastname@example.org"
You can set the default SMTP server by defining it in
/etc/mail.rc with the following line: