The API was recently updated, and is now much quicker, and returns more information about which MX record was tested, etc.
Using this API you can easily check if an e-mail address is valid. The API will quickly check MX-pointers and find out if the mailserver is valid.
This way, you can easily catch typos in e-mails in signup forms, for example "someone@hootmail.com" which will return a “valid: false” since that domain doesn’t accept e-mails.
You can also block temporary e-mail addresses or block disposable mails! The API has a built in blacklist and multiple heuristic methods of checking if a domain is used for temporary disposable e-mails.

The API response now includes things like:
reason: Blacklist
mx_host: mail.mailinator.com
mx_info: Using MX pointer mail.mailinator.com from DNS with priority: 10
mx_ip: 23.239.11.30