That will work for some mail servers. Others will accept all mail, and later generate a bounce for it. qmail behaves this way, for example. If you tried using this, VRFY, EXPN, and also checking that the domain still exists, you might be able to trim some users from your list, but not all. It's probably easier to just send the mail and watch for bounces.

For bounces, BTW, you should consider using something like VERPs. The general idea is you set up a catchall mailbox for bouncetest-*, then send out each message with a unique sender that starts with bouncetest-. qmail's VERP implementation encodes the recipient email address into the bounce address, although you can put anything you want there. Then when you get a bounce, you know what message you sent caused it. It can be surprisingly hard to figure this out otherwise, either programatically or by hand.

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