Email Scanning Rule Not Applying

Hi All,

I created a business application rule to allow SMTP access to my exchange server using the email servers (SMTP) template. Below are my settings:

Now, on my LAN network i can get a connection on port 25 to the SMTP server by using the external IP, but if i try to access my SMTP server externally it won't connect.

I have created another business application rule but this time i used the DNAT/Full NAT/Load Balancing template and configured it the same. Now i can get access on port 25 internally and externally.

Have i missed something? It seems when i add SMTP scanning into the equation it blocks access externally. Without external access my SMTP server cannot receive emails. 

Regards

Oliver Knights