At present the app is dependent on you, the user, to regulate yourselves by not sending more than 100 messages in an hour. An hour is surprisingly long time when you're waiting to send more messages and the occasional Force Close report supports my suspicion.
I'm on the verge of a release that will keep track of how many messages you've sent in the last 60 minutes and present a dialog to keep you from making a mistake.
Trust me; this isn't the end of the message limit issue. I just want to eliminate as many force close issues as I can before starting any new feature development. I also have a lot of refactoring to do. Quick and dirty means I ended up with a lot of "work" being done on "screens." That has to go ;)