Using older versions of Outlook for Mac for personal email and calendar
Same issue for me - many messages are getting stuck in drafts unable to send, despite other outbound emails sending fine at the same time.
Haven't been able to narrow down the cause, but once it decides it doesn't want to allow a reply in a particular thread, it wont ever allow it within the desktop app.
The only workaround which has worked out of all the above for me is to allow the message to enter Drafts, then head to mobile, find it in drafts and send from there. Seems to work.
Not a workable long term solution.