Short, concise description of the idea
Allow users to send messages to other users by use of special page.
Full description of the idea
Allow users to leave messages for other users. There would be a page where a user can go, type in a user's name, and type in a message to be left for the other user to retrieve when they log into LiveJournal.
Not actual email, only a way to send a message (or comment) to a user, without having to reply to a post. The user would be able to read these online, and delete them, etc. As an example, RateMyFace has a similar usermail service.
An ordered list of benefits
An ordered list of problems/issues involved
An organized list, or a few short paragraphs detailing suggestions for implementation
I would be glad to help out with this as much as I can. I just figured it'd be too big for me to do on my own.
Update: This post is changing as I receive comments in order to clarify.