How to use mx lookup.smt and whois lookup.smt server type? Sometimes not work correctly.

If you select this SMTP relay server, it search email DNS MX records and giving back email's ISP. Some e-mail address can not get mail server correctly. If you use relay server.smt, you couldn't get wrong answer, always send back 'succesful'. If you send the mail and recipient don't get it, it is something wrong on DNS relay server. See free Resolving Name Server homepage what I use in program.