Out of curiosity, when did you launch the site? Did you do any marketing or was it natural growth? I ask because the temporary inbox market seems pretty saturated already.
I didn't do any marketing, just mentioned it to a couple of friends. Apparently those friends know some crazy people because in ~24 hours it was on the front page of Digg, Slashdot, and Yahoo Tech.
Since that initial spike of traffic, and the settle-out afterward, it's been growing slowly but steadily ever since.