Email flow issue with following error in SMTP/MTA log: 451 4.7.0 temporary server error

If you are using Email Security and unable to send emails to your Exchange server 2013 and if you see the following error on the SMTP or MTA logs:
451 4.7.0 Temporary server error. Please try again later. PRX5.

This is a Known issue with exchange 2013 server.

By default, the standard front end receive connector is bound to all the IP addresses on the server as seen below, this can cause issues with DNS lookups.

To fix the issue please follow the steps as mentioned below:

1. Navigate to Default fronted |Scoping.
2. Click on the pencil sign and edit the IP addresses from All Available IP to its Specific IP ( as shown below).
   
   
   
   
   
3. Click Save .
4. Now go to the hosts file located at c:WindowsSystem32DriversEtc.
5. Edit the host file and add entry for Exchange 2013's IP and its host name.
6. Save the file and reboot the server.
   
   
   
7. Once the Exchange server comes back, try to send an email from outside and you should be able to receive it.