Using Classic Outlook on Windows for personal email, calendar, and contact management
A quick fix I found is to (1) go to your outbox, (2) right click the email that is stuck, (3) move the email to drafts, (4) then find the draft email, and (5) hit send again. May not be a long term solution, and very tedious (would much rather just have the rule work), but that at least fixes the immediate problem.