Sorry, I've given you a bit of incorrect information....
First, set $FROM_USER back the way it was originally. That's only used for alerts, and if alerts were working before you should leave it the way it was.
The default looks like this:
The reason you're getting the message about SendMailFOption is that if you specify this, you also need to specify FromAddr.
// ** OPTIONAL **
// Set FROM_USER to the email address that will be the sender
// of alert/error messages. If not specified (comment it out),
// formmail.php uses "FormMail@SERVER" where SERVER is determined
// from your web server. If set to "NONE", then no sender is specified.
//$FROM_USER = "email@example.com"; // this line is commented out, by default
//$FROM_USER = "NONE"; // use this to show no sender
Here's what I recommend:
<input type="hidden" name="mail_options"