With the Byteplant Email Validator plugin you can easily verify with a real-time live check if an email address really exists and is valid (https://www.byteplant.com/email-validator/).
Very simple antispam plugin.
Users do not need to enter any captcha, working completely on background.
No need to administer: just install the plugin and site will be protected.
Fully compatible with Contact Form 7 and Formidable Forms*.