Hi, Walt,
To be honest, I've rarely used Loremasters- I generally left it up to the players to bail themselves out of the troubles that they generally got themselves into!
I always felt that the Loremasters were quite rare characters- most normal people could probably go their whole lives without (knowingly) coming into contact with one.
(Mind you, they used to run afoul of Servants of the Unlife with embarassing regularity.)
As for a secret language, it doesn't necessarily need to be secret, there are spells that will get around that type of thing (Lore, etc.) But, there are ways of obfuscating text, substitution, cyphers, etc.
But, as for a postal service, again I've not really used it- just taken it as read that any large city will have one- and it's pretty reliable. You just have to look at things like the Pony Express and the Royal Mail, they were reliable (or as reliable as they could be.) Certainly, within a Realm like Rhakhaan, it would be comparable in security to the Royal Mail.
And, the advantage of sending a letter c/o The Palace pretty much ensures that it's going to be fairly safe in amongst the large number of other letters they will recieve. (And, the mail for the Palace is sure to be well guarded.)
Mail for other areas would be more problematic- but there is almost certainly going to be some sort of service between places like Haalkitaine, Lethys, Sel-Kai/Eidolon- possibly on a Sky-Ship so, again security is likely to be tight.
The bad guys have got several problems, the biggest being which specific mail delivery is the letter they are interested in, going to be going in? They hit a couple of deliveries and security will be tightened up. (The Loremaster could also send several copies of the letter- just posting them at different times.)
For particularly sensitive documents, they could simply pay someone they know to be reliable, to take it where ever it's got to go.
Hope this helps,
All the Best,
Kevin.