Email Helo Tool:

Your email program becomes an SMTP client, connects to port 25 of your mail server (usually the SMTP port) and -- says EHLO.

Computers, in the end, are only human and what counts is that it wants to be polite. Actually, it does not attempt to be polite but to use later additions to SMTP that have brought about two flavors of the later HELO command (SMTP command generally consist of four characters). The only way to validate if an email address is correct is to talk directly with the mail server you are trying to send to. If the server returns that the user is valid and that the machine will accept an email for this user. This is the best way to know if the server is answering requests and responsive.