Is a PHP class that can be easily used to verify an email address and make sure it is valid and does exist on the mail server.
Some mail servers will silently reject the test message, to prevent spammers from checking against their users' emails and filter the valid emails, so this function might not work properly with all mail servers.
You server must be configured properly as a mail server to avoid being blocked or blacklisted. This includes things like SSL, SPF records, Domain Keys, DMARC records, etc.