This is how you can send a message from the commandline using the mailx (mailutils) package.
$ mailx -s "Re: My mail subject" -r "<email@example.com>" firstname.lastname@example.org < contents.txt
Note, however, that I was unable to specify the sender's name.
-r "Foo<email@example.com>" and
-r "<firstname.lastname@example.org> (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.
My workaround was to create a brand new user account on the system using
sudo useradd -m myusername -c "My Name" and switching into that account while mailing out.
Hope this helps.