Yeah, most confusing thing is that deleted threads recipient list still contains player who deleted it, and you can't say, will he receive your next message or not...
Not sure how it's implemented serverside now, but probably each player have a list of threads to receive (subscriptions). Threads are added to this list only when someone start new thread/player joins FS, threads are removed from this list when player delete thread from his mailbox/old threads pushed out by new threads due to limited number of subscriptions.
So we need few things:
1. ability to refresh subscription for FS-wide thread for all FS members (may be limited to Archmages/Mages) that will force players to receive thread even if they have deleted it.
2. ability to resubscribe to deleted threads (should show deleted threads1 captions for last X deleted threads and a button to resubscribe).
3. ability to check the list of current subscribers for particular thread in addition to original recipient list that is seen now.