MailBox Posting - Control Category

This is a request for future development - Not really sure how easy this would be.
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We have just started using mailbox posting for our contributors. This is designed for simple quick posts where they just want to include some text and maybe an image (Everything works well
The only thing we would like is for our contributors to be able to select a category. We have set the Default category, so all the mailbox posting posts go into this category. However we have a number of other categories which these contributors post in.
It would be nice if they could specify the category in the subject line of the email. Currently we have
SUBJECT: [news] ...this identifies it as post ....then the title of the post ie My Latest Post
so it would be great if we could have
SUBJECT: [news] [category name] ...This would be the category in which to post the post .... then the title

I hope this makes sense???
Kind regards
David