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, for example.
Mailcheck uses an open-source project and you can access it on GitHub.
Enable Mailcheck
If you already have Mailcheck installed, select Yes or No to enable or disable it and then select Save Config.
Magento 2.0
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 > DOTDIGITAL> Configuration.