Easy way to check it to use telnet.
To check if you can communicate with a internal mail server do the following.
from external pc.
telnet (Ip address of public nic on router) 25
eg. telnet 192.168.0.55 25
for a pop3 server connection change 25 to 110 and so on.
If you look at your firewall rules there must be a rule that says something along these lines.
"Allow in coming traffic from anywhere on port 22 to local ip address."
Now i assume that your internet router has a ipaddress on the local nic and one on the ppp+ port. So lets say your internal ip address is 192.168.0.254 and the ppp+ address is 42.253.200.110 (Assigned by your isp). now to allow a connection on port 22 (ssh) from the outside world you need to make a rule in your firewall that says this.
allow tcp traffic from anywhere on port 22 to 42.253.200.110. (Not sure what router you use).
If you have a internal server with ip address 192.168.0.115 that is separate from the router running a vnc service you would do something along these lines.
allow tcp traffic from anywhere on port 5900 to internal server 192.168.0.115
Then always remember to deny everything else from coming in as the last rule.
Hope this helps