It's not in the application config, it's in the folder properties. Tap and hold on the folder, and scroll the menu way to the bottom. In the properties screen you see the option to check for email, synchronize, and folder processing. Might be one of those.

If it's crashing at other times, try going offline then write your message, that will at least tell you if it's having issues downloading in the background. You may want to set up MH or mbox folders so you at least have a queue to put your messages in.