We apologize for the instability of composr.app and appreciate your patience. We are working on the statistics addon and trying to find an optimal way to store and render data. Unfortunately, we have yet to find a solution that can handle the traffic (and therefore, tens of millions of statistical records) of composr.app. We're working hard on one.
1) We can't do it on the client-side, unless we now do address validation using AJAX
2) People might get annoyed when testing using junk addresses
Also of note that during signup you have to double enter your e-mail already, which will pick most errors.
I think together this makes the benefit of the feature marginal, more useful just for newsletter cleaning.