The tool performs SMTP validation when the SMTP checkbox is checked and vice versa. The SMTP validation involves establishing a socket connection with the mail server and simulating the sending of an email. This being the most trickiest part of the entire validation process can return false negatives cause of certain settings at the mail server end(Most mail servers have some sort of checks to block spam). The tool has returned successful SMTP validation checks for the following domain names :
google.com, yandex.com, yahoo.com.
If your list to be checked includes other domain names there is a high chance of getting false negative results (Soft bounce warning mostly) for even valid email addresses if the SMTP flag is set.