This is a hosting/server issue that indicates your sendmail has not been configured for use correctly by your hosting company.

You need to contact your web host and ask them why your server will not execute the Sendmail program. 

While it is easy for web hosts to say that it's a problem with the script, the error indicates a "refusal" from the server (assuming Sendmail exists at the /usr/sbin location), not anything the software is doing wrong.