In this most recent post to the PHPClasses.org blog Manuel Lemos talks about invalid email addresses and shows the use of this package to evaluate them.

When you take users' email addresses, for instance in a site sign-up form, there are great chances that the addresses may be incorrect because of a typing mistake or it is not possible to deliver the message to the specified address for some reason. This e-mail validation package can detect and prevent that users enter incorrect addresses even before you accept them.

He starts the post with a list of six types of invalid email addresses including everything from simple typing mistakes out to temporary rejection from "gray listing". He shows how set up the class and briefly covers some of its methods and what they do. Also included is an example if it in use to validate the address. There's also a brief section at the end talking about using OAuth to work around users not wanting "yet another account" or to share their details with an untrusted application.