Mailcheck is a plugin that determines the validity of email addresses that customers are typing into forms and suggests valid domains when invalid or incorrect ones are being entered (due to a typo, etc.) Through using Mailcheck, you can experience a significant reduction in bounces caused by such signup errors.
Mailcheck uses an open source project and you can access it at https://github.com/mailcheck/mailcheck/.
If you already have Mailcheck installed, click Yes or No to enable or disable it and then click on Save Config.
Please comment underneath with any questions you may have.
The above also applies for Magento 2.0, with the only difference being that the 'MailCheck Settings' section is found by going to Stores > Configuration > DOTMAILER > Configuration.