Question
My contact form is being abused by bots to send out spam. What can I do about it?
Answer
The best method to combat bot abuse on your contact form is to implement a Captcha system. Captcha creates a challenge-response test to prevent bots from using the mail form to send out spam from your users' account.
A sound Captcha system you may consider is reCAPTCHA, which is provided by Google.