Unable connect to webmail via Port Forwarding

Question

Dear Export. 
 I'm new in handling Sophos Firewall and i hope i can get some advise. Below is the scenarios. 
 
 Background 
 
 Web user connect to webmail server via port forwarding. 
 Port forwarding from example Port:12345 to Port:80 
 
 Issue 
 
 Today, User log a report and informed us that they unable to load the login page to our mail server. 
 
 Workaround 
 
 Immediately, I tried to telnet<domain_name.com>12345, I received Connection Failed. 
 I performed the telnet testing again after i restart the mail server and the result are the same 
 
 Question 
 
 From Sophos, May i know how do i check whether the following port:12345 was not blocked?

apijnappels · Answer

If you need to connect from outside on port 12345 to your inside server on port 80 you need a DNAT rule 
 
 Traffic from: Internet IPv4 (or any) Using service: port 12345 (needs to be created) Going to: External (Address) (be sure to use the (address) item otherwise it will not work). Change service: http Change destination: Your internal server 
 
 Be aware tough that when you need to access this from inside you will need a full NAT rule when you access port 12345 on your external interface from the inside.