Hi, I love your plugin but wanted to make a feature request, which could set this plugin apart from POSTIE (big time!).
In multi-author blogs, we all know there are many users that have very limited technical skills. Therefore, requiring them to use shortcode to advise where they want to post things might be overkill.
Wouldn't it be simpler to create multiple email addresses and assign each of those one category?
and so on.. I think this could increase usability and since most hosting accounts come with unlimited email addressses these days, this could be a nice feature. And it's a lot easier to share a list of email addresses with authors than to explain them shortcodes.
Anyway just a thought, all the best of success to your plugin!