In theory, you could connect to the server and use this SMTP protocol to attempt to send mail to each of the users (but abort the connection before any e-mail is actually sent). In theory you could also use the VRFY command for this, but it's often disabled.
This looks to me like a question that might be asked by a spammer, so let me give a couple friendly words of advice:
(1) Don't actually do this unless you have explicit written permission from the owner of the SMTP server. Otherwise you will look like a spammer, your IP will be added to the common spam blacklists, and the abuse department of your ISP will be notified.
(2) If you can't figure out the SMTP protocol well enough to understand how to do this, don't even think of attempting it.