Please login to the MailEnable mail server and follow the steps listed below:
1) Open the MailEnable Administrator (Start -> Program Files -> Mail Enable -> MailEnable Administrator)
2) Go to Servers -> Localhost -> Connectors -> SMTP. Right click and go to properties.
3) Click on the reverse DNS blacklisting table. Click on the "Whitelist" button at the bottom of the screen.
4) If it is not enabled already, check "enable Whitelist'. Now, add the mail server IP address which one needs to Whitelist.