I'm running into an interesting scenario with PMs. I provide moderation and administrative feedback to members via PMs. Things like signature images that are too large, avatars that are inappropriate, warnings for inappropriate behavior, etc.
If the user has read the PM, then unsubscribes from the PM without replying, there is no record in the PM of that original participant. The 'Participants' box only shows my ID, and the 'unread by: none' indication is of no help.
With a large number of PMs over time, there's no way to remember who you sent the PM to in this situation, making future moderation awkward and repetitive, and removes any 'documentation' that may be required for future actions.
I recommend adding some archiving of all of the participants who were active in the PM, even if they have unsubscribed from the PM. One possible implementation would be to reverse the 'Unread by:' indicator, turning it into a 'Read by:' indicator. Even with that implementation however, the user(s) could delete the PM without reading the PM and you'd be left with no indication of the original participant(s).
