Check the Archive Settings!
If you delete old emails to archive them, check Deleted Items archive settings (right-click > Properties… > AutoArchive). Keep the emails around long enough to have them when you need them. 12+ months recommended.
You can save deferred emails any reasonable place in Outlook and find them again easily. A couple of good options are Deleted Items (see side box) or a single folder you create for filing everything.
To see or Reply to the original email, open the folder where you save old emails, The original email opens in Outlook, ready for you to reply.