JPEG, DOC, AVI, PDF, etc. Persistent, auto-syncing private conversations akin to discussion or newsgroup. Allows for person to person or group threads. When working on projects, sometimes you just want to post a short idea, encouragement or message, but it doesn't deserve mail. Posting may or may not require response by others in the group, which would be a flag in the posting.
Users only see and sync on threads for which they are members.
Also gives a shared traceable record for project participants. Has automatic garbage collection for team, but members can elect to store any thread offline. Simple concept to solve a ubiquitous problem. ISP's and portals don't do a very good job running mail for small business and individuals. Distributed computing clients are used to store and forward mail for users of the network, or as a host for other domains.