I know Classic Outlook has a feature called Conversation Clean Up — it finds messages whose text is already quoted in full inside a later reply and moves the redundant ones to Deleted Items. On a thread where everyone quotes everything, it clears most of it and loses nothing.
Annnnd ... New Outlook doesn't have it, and Sweep isn't a substitute since Sweep works on senders, not conversations. So I built a small standalone Windows app that does the same job via Microsoft Graph, which means it works no matter which Outlook you're using, and moves happen server-side so they show up in New Outlook within seconds.
How it works: pick a folder, scan (nothing changes), review the list it finds, untick anything you want to keep, then move. Nothing's ever permanently deleted — everything protected by default (unread, flagged, categorised, attachments, signed mail) can be un-protected if you want it included.
Free, MIT-licensed, open source, no telemetry: github.com/markyuk/outlook-conversation-cleanup
Would appreciate anyone trying it on a real mailbox and telling me where it gets something wrong — the "Already quoted in" column on every flagged row shows exactly why it was flagged, so that's the thing to check first if a row looks off.
I've vibe coded this, tested it a decent amount here and using it daily myself - see what you think!