I don't think that particular solution would be good from a user experience point of view, but it is indeed a nice idea to filter out domains that you have received emails from (and are not deleted or in the spam folder).

However, there are ways around this too. I think the fundamental mistake was to allow (all?) unicode strings as urls. However, I can't come up with an elegant solution on the spot (since it would be unfair and unpractical to use ASCII for this).