Today email address are asked by many websites for signing up for there services. How do they validate email address? May be using code which ignores RFCs. Say if your ISP has issued you email address which flouts RFC conventions and it is invalid. If you can send and receive emails on that why does it matter its invalid?
Here comes is_email originally developed by Dominic Sayers written in Java, C# and PHP which validates email address.
You can also use it with jQuery and RequireJS. Find details on how it can used in is_email Github repository README.md file.
If you find bugs please open issue on Github or feel free to fork and send pull request.