You may need to add one more HIDDEN INPUT field in your form. WordPress requires a NONCE field and generates a code for this each time the form is submitted by a new user. Here is a sample line of code taken from one of my projects:



You would then need to check for the existence of this value when the form is submitted to ensure it is coming from your own website. This is a security feature so that people will not SPAM your contact forms from other websites.