In most cases the mailboxlayer API's SMTP check is entirely accurate when it comes to detecting whether or not an email address actually exists. We are saying "most" cases because certain mail servers tend to suddenly behave in unexpected ways and cause trouble out of nowhere, which may decrease the effectiveness of "pinging" a mail server via SMTP at times. Combined with all other mailboxlayer validation tools, however, there is no better way to separate low-quality addresses from high-value email contacts and to successfully keep fraudulent users out of your system.