Ubuntu :: "-r" Flag For Mailx Not Working?

I recently set up an email server at work on a ubuntu box, running exim4. Everything seems to be working fine, but when I try to change the "from" header in the message, its not letting me.

I wanted to send a message from the root user as "admin".

I assumed this would be easy as:

Code:
mailx -r admin recepientname

but for some reason the mailx program is claiming that "-r" is not a legitimate tag, when I try it tells me:

Code:
mail: invalid option -- r
usage: mail [-eIinv] [-a header] [-b bcc-addr] [-c cc-addr] [-s subject] to-addr ...
[-- sendmail-options ...]
mail [-eIiNnv] -f [name]
mail [-eIiNnv] [-u user]