This is how you can send a message from the commandline using the mailx (mailutils) package.
$ mailx -s "Re: My mail subject" -r "" bar-recipient@example.com < contents.txt
Note, however, that I was unable to specify the sender's name. -r "Foo" and -r " (Foo)" did not work (and neither did the versions with the email addresses with the <> removed. What was sent instead was the username of the commandline account.