Problems sending mail: Difference between Mail and Mailx?

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http://www.unix.com – Whats the difference between mail and mailx? I'm trying to troubleshoot a problem where I can send mail from server A with this Code: `echo $MESSAGE | mail -s "$SUBJECT" -r $FROM $RECIPIENTS` command but executing the same command from server B throws me this error (Both servers are RHEL) Code:     mail: invalid option -- r     Usage: mail [-iInv] [-s subject] [-c cc-addr] [-b bcc-addr] to-addr ...                 [-- sendmail-options ...]           mail [-iInNv] -f [name] &n (HowTos)