I'm using the post by mail plugins.
It's possible send post by mail to a specific category?
This guide is valid?
If [n] is present anywhere in the subject line of your email, the email will be filed under the category numbered n. Otherwise, it will be posted under whichever category is selected for "Usual Category" in Step 1. Example:  This is a Test! would be posted as "This is a Test!", filed under General.