I'm trying to remember when the first international postal system was set
up. It was run by a prominent European noble family, I believe the company
was called something like "Sturn and Taxis" and it survived a couple of
centuries until Napoleon when it was eliminated. I believe it started up in
the 1500s. So it is more of a Renaissance phenomenon than a medieval one.
Some models for Ralios do look to be post-medieval, so maybe a postal system
would be an appropriate idea.
To make it more Gloranthan, it could be given some religious structure.
Perhaps it is a special branch of Issaries, in his Communications aspect.