All good hosts prevent a form on a site they host from sending an email from an address that doesn’t match the site’s address, this is to prevent a site they host being used to send spam.

For example, if a site has a name of www.mywebsite.com and the email address specified as anything@mywebsite.com it will work but if its specified as anything@anotherdomain.com it will fail.

The solution is to create an email address in your site's cPanel for sending emails from the form.

Note: The destination email can be any valid email address.