Here’s the situation:
You’ve managed a WordPress.com blog and it looks kind of neat, it has a lot of followers, and your heart grew attached to it. But you’re too long in this game to ignore the obvious limitations of WordPress.com, and you have moved to a self-hosted site.
But you want to keep the old blog going…
This can be done effortlessly using the posting by e-mail function of WordPress.com. IT not only allows you to send HTML E-mails (provided the markup is correct), but it also accepts a number of shortcodes like so
[tag cool, e-mail, posting, geek]
[category WordPress]
[delay +2 hours]
[more]
[geotag on]
[publicize on]
Check out the full list of possibilities here: http://en.support.wordpress.com/post-by-email/
So, when sending these e-mails with a plugin from a self-hosted WP site, you can create an action hook to add the tag and category shortcodes. Will look into that next time.