Remote Assistance refuses to communicate via the internet, what's wrong with this picture?

[Tom]

When attempting a 'Remote Assistance' session over the internet, I receive
the following message; "Remote assistance connection could not be
established because remote host name could not be resolved"

RA works fine on my two home networked computers but refuses, with the above
message, on three different remotes. Request have been established via both
email and Messenger methods several times all resulting in the same
response.

The above message, quoted verbatim, is not found in the knowledge base,
which I find a little disconcerting considering this is a tool for
assistance/help, not something designed to require a CE at each end.

Help please.
TIA
Tom

 

[Robin Walker]

When attempting a 'Remote Assistance' session over the internet, I
receive the following message; "Remote assistance connection could
not be established because remote host name could not be resolved"

The most common cause of this is that the Novice requesting assistance is
behind a NAT router, and UPnP functionality is turned off or absent.

If the Novice is behind a NAT router, then, for Remote Assistance to work,
- the NAT router must support UPnP;
- UPnP functionality must be turned on in the router configuration;
- in Windows Firewall, an Exception must be enabled for "UPnP framework";
- in Windows Firewall, an Exception must be enabled for "Remote Assistance".

There are no special conditions at the Expert end.