You must specify direct recipients with -s, -c, or -b.
This is obviously an error message from mail and not the script itself, since your script doesn't use flags. So check your mail command. Of course evo2 has already pointed out the error above.
You can also run your script with "#!/bin/bash -x" to get a verbal output of all the substitutions it's doing. This should clearly point out your variable typo.