Ubuntu

You could try a probe from external using tcptraceroute and check if it's being blocked somewhere on the way, but you'll probably get a definite answer if you call your ISP. That way you could also ask them if there's any other way for you to access your machine at home (sometimes they can get coaxed into opening a port).

SSH, FTP, and HTTP can use other ports if necessary, not sure about VNC, but from a security view I'd suggest to enable VNC only for localhost and using an SSH tunnel to connect.

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