Given an email address I would like to not only verify the validity of the domain but also perform a lookup that checks whether or not the user's account is still active. Is this possible?
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Probably not - many / most mail servers turn off the ability to verify user names because it was considered too insecure.
It also invited connections from spam bots which would simply try to verify every single possible combination of names, and thus, enumerate a complete list of valid accounts on the mail service.
There are a number of other problems as well, for example, maybe I accept mail on my mail server for every single possible address, and then bin the ones that don't exist, that way, I never reject anything and don't add to the spam rejection rubbish.
Here's a nice summary of why address validation is usually considered bad.
While I agree with Zoredache and EightBitTony that the existence of such a tool is probably not a good thing, the fact is that they do exist: