i already install zimbra, but i get error, DNS error resolvig from MX from yyy.com

my topology

Server -- switch -- router mikrotik -- ISP

I dont have local DNS and i use DNS from the ISP
i already buy a domain and want use own mail server with IP Private (Local IP) but can access from Public,
in my domain i also create mail.mydomain.com in MX record.

someone have the same problem and solved when they use Split DNS,

i already use split DNS, but still have same problem,

Can some one help me to get out from this problem?

Many Thanks,