Manvendra make sure you have the A record added into your domain name provider (name.com, namecheap, godaddy, etc) add a A record that points to your IP machine address and then create a mail.domain.com, on that field you will place your mail.domain.com and after all is done you should solve the dns properly. My advice, use the DNS manager in the aaPanel before you configure the mail server on it. By the way... Takes a few minutes to a few hours to update DNS, you can check your configurations also at dnsinspect.com, if all is done correctly you will be assigned with a class A.
