I've seen a couple of snippets for validating an e-mail address. These went so far as to check that the mail server really exsists and another ( I think on Zend ), even checked to see that the user really existed on that mail server.

Question is, does any one know how reliable this would be? The one snippet mentioned, one drawback being if the mail server was down when it was checking. Course if the mail server was down, you would'nt be able to send them mail anyways. The other thought was could any mail server stop or prevent the script from checking if a user and/or mail server was "real" ?

Also, a dumb question, what is the correct port and can it ever vary from mail server to mail server?

Benifits for me would be not to waste time on bad address's. I would think even validating the mail server would uncover some phonies.

Any one have any comments or thoughts?