Strange... I know the original mail app will do that, but Modest hasn't done that to me at all.

Hate to ask... but are you 100% sure that you're using the Modest Mail app? The original osso-email app shows that behavior. And oddly enough, both are placed in the menu as "E-mail"... which is annoying as all get out. I've seen the behavior you've mentioned in the original Email app, but not Modest.