Best answer... Call to your ISP or where you dns records are and request to change your PRT record in the DNS for your public ip address. You have to add a name, for example: mail.mydomainname.com
After that, add the same name of the ptr record for your domain name inside the section of domain setup in Mdaemon, host name section, SMTP Server name.
Wait for one day for DNS replication. Done.
Pablo Solares