I wouldn't be surprised if some of the Dunmer have started to trickle back to their ancestral homelands as pioneers.
The loveletter not only takes place on Morrowind, but on Vvardenfall. According to it, Kwama miners have survived and made underground cities that will last until the end of the 5th Era (Also when the Kalpa ends, and everybody dies)
Some of the forumites have made it sound as if Redoran will just move the bulk of their people to the mainland, but I have the feeling the Dunmer have been so decimated that the House system won't recover. Though come to think of it, there are a couple of Houses mentioned in the "Loveletter", like Sul and Jaroon.
Who said that? As far as we know, Hlaalu, Indoril, and Redoran were completley destroyed. Dres was torn apart by angry Argonians, and Telvanni are hiding away at Port Telvannis, waiting out their immortal lives until Morrowind is habitable again.