@zogstrip has been working on implementing this, I have been working lately on improving group support.
After quite a while contemplating this problem we have decided that giving messages a category is a bad idea 
. There is just way too much fallout.
Instead we are looking at beefing up group messaging support with the following changes, that can be rolled out incrementally.
Allow groups to have an incoming email
If we receive any email to the “incoming email” address for a group:
- Create a staged account for the user if user does not exist in the system.
- Create a new message targeted at the group. (This is already supported by the messaging system, not much new code needed)
Allow users to untangle messages targeted directly at them vs ones targeted at group
Add a new section here:
- My Groups: Messages not targeted directly at me (but targeted at my groups)
Allow group members to view all messages on group page
This is a pretty simple change, allow a grouped view of all messages to a group
Allow users to specify default “loudness” of notifications for group messages
- Allow users to specify default watching/tracking/muted for messages targeted at group
- Allow user to opt in to “blue” notifications vs “green” sticky notifications for postings to the group (needs some discussion first)
These changes drastically reduce the amount of time we need to work on the feature, heavily simplify the implementation, can be be delivered piecemeal and fit very nicely into our theme for the upcoming release.