Please remove it and just punish the abusers, thanks.
Speaking as someone who has played a lot of Dominion, I think it is worth considering removing the cooldown or replacing it with something else. Sending wars could be annoying but they weren't all that common. It does strike me a little like the case of the majority of decently behaving players suffering from the fallout of the mistakes of the few.
A suggested alternative: could it be scripted so players who make a lot of sendings in a row get a cumulative chance of getting a Sending migraine so they can't make more for the rest of the reset? So the first two are okay, but the third inside one rest period has a 25% chance, the fourth a 75%, just as an example?
Or something like it. It could be getting baleful polymorphed by the town crier if he gets tired of you bugging him. "Stop cackling like a chicken on my time!"
Or-
Simply let the Sabuthans have a means and way to limit the Sendings of others in their oppression just like the Armada can.
Revoking sending rights can work just as easily to halt abuses.
I dunno, now I kinda like the idea of the town crier going on a Baleful Polymorph rampage when he's had it. >.>
Considering some sendings are actually long speches that don't fit in a single conversation bar, i'd say removing it would be very good.
Using the Mythallar to make a sending, that's easy, it still works the way it's intended. Using that broken piece of crap they have in Avner's Tower to make a sending? That seems more complicated, a lot more complicated. Up the price to 25 gold, and problem solved.
A full breakdown of the docks sending thingy would be in order. Would take care of all the sending wars and the cooldown problem.