Email Checker Basic tries to verify that an email address is legitimate, without requiring you to send an email.
The program couldn't be much simpler to use. Type your address in the box, click "Check Now", and wait for the verdict: easy.
And if you want to understand how Email Checker Basic came to its conclusion then reading the contents of the Interaction box will tell you more, as this details its conversation with the remote server.
There is one small problem here, though: verifying email addresses is more complex than the authors seem to believe, and while the program worked just fine with some servers, with others it would always confirm an address as valid. Even if we'd just made it up.
This doesn't mean the program is entirely useless, though - you just have to keep this problem in mind. And if you want to check firstname.lastname@example.org, try a couple of nonsense addresses with the same domain, first: email@example.com, firstname.lastname@example.org or whatever. If you can get Email Checker Basic to tell you that one of your nonsense addresses doesn't exist for a domain, then you know verification is working, so you can test the real address and trust the results.
(And if you're still not sure that it's worth the hassle, you might have a point, but at least the program is portable. There's no adware, nothing to install, if it doesn't work out then delete the download and carry on as before.)