Mail Servers doesn't accept my Emails

I am getting the following reply when sending mails to GMail:

"Our system has detected an unusual rate of unsolicited mail originating from your IP address"

I have the following setup (3 VMs):

DC (Windows 2012, 1 NIC (Private)

Exchange2013 (Windows 2012, 1 NIC (Private)

NAT (Windows 2012, 2 NICs (Private, Public)

I am able to send Emails to other servers like HushMail.com, however I couldn't receive from them

I got a domain name from Godaddy.com with the public IP.

Where exactly to put the public IP, is it in DNS, NAT or Exchange??