You need to use the catch all IP 0.0.0.1 for the outside address. That is unless you wish to restrict it to a specific outside IP.
You also need to create a firewall rule to allow this incoming connection.
This may give you the general idea.
http://npr.me.uk/forwardports.html
There is a cli command which will assign a PF rule which was previously created in the GUI.
Sorry I don't have a Technicolor router running here to dig out that command, but it shouldn't take much finding.
Edit:
One of the reasons people have problems with the routers GUI port forwarding is because the device is on a static IP address. Try a dynamic (DHCP) address for the device and in the routers network settings tick "always use the same IP address"

Ok I worked it out by looking at a backup file and some guesswork.
You don't use "nat mapadd", what you do is this:

service host add name="Mail Server"
service host rule add name="Mail Server" protocol=tcp portrange=25-25
service host assign name="Mail Server" host=10.0.200.11 log=enabled

And, in case you want to remove it

service host disable name="Mail Server"
service host delete name="Mail Server"

Quote from: npr

One of the reasons people have problems with the routers GUI port forwarding is because the device is on a static IP address.

I have a static IP address but I don't think that's the problem, as I have it working now. The problem is the router CLI is very badly documented (a command list doesn't suffice without information on how to use them) and the web gui just plain sucks. I mean... why on earth does adding a "game and application sharing" rule cause the router to drop its DSL connection? And why does adding a "game and application sharing" rule dropdown not contain all my hosts in DNS and why does the box to type in an address not always appear. Because it sucks big time. Why on earth is it called "game and application sharing"? Sorry **rant over**. I just wasted over two hours on this