First off, I love this plug-in. Have used it several times on several sites with NO issues.
To be a specific as possible:
I am using this plugin on a site hosted on BlueHost.
I have 5 forms.
1 is Contact Us which works, as long as it's set to senf via php instead of WordPress.
The other 4 give the error "ReplyTo Header Missing or Malformed"
When attempting to send. The webmaster is not receiving an e-mail.
The sender IS receiving a confirmation.
The only differences I can see is that the contact us doesn't have any drop-downs, radio buttons or check boxes which the other 4 forms have.
Was on the phone with Tech Support for over an hour- no luck.
The site is http://www.adaraspa.com