Querying to see if a mail address exists can be a sure fire way of getting your
server blocked. it is a common way for spammers to find addresses by running a
generator against an inquiry loop.
If you are asking because you want to run a mailing list etc, you need to use
a double-opt-in method for adding addresses; and make sure you never send email
to an address that does not confirm validation. Never add addresses manually,
only allow the end-user to add them and then make them verify it.
As long as your mail server has RDNS, SPF, is not an open relay, and has valid
domain MX information, mail servers won't care of you send through the
occassional dead address. What matters more is how you handle dead addresses
when you find them. ([i]i.e. remove them immediately[/i])