Data for MailRoom ownership

When discussing our recent MailRoom project in #1604 (closed), I noted that it may be possible to use data to allocate ownership for MailRoom. Let's use this issue to explore if this will work.

I know that there are many types of incoming and outgoing mail that flow through MailRoom and that each of these types will belong do a different feature category or user group.

Please could we look at one average week's worth of data to see if there are any types that stand out as the heaviest users?