In some cases, you can ask the receiving MTA if the user is valid and it may tell you. In many cases, however, there's no way to know.
And because confirming the existence of valid e-mail addresses tends to increase vulnerability to attacks and spam, more and more email servers are taking the prudent step of disabling this functionality. In the future, I would expect it to be impossible to ask whether an address is valid and get an answer.
-- Rodolfo J. Paiz rpaiz@xxxxxxxxxxxxxx http://www.simpaticus.com
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