Mail: Occasionally messages will not send, but will go to Drafts instead

Unfortunately Apple Communities has auto-locked the thread where we were having a productive discussion on this - see https://discussions.apple.com/thread/256106911

It appears that there may be a bug in how Apple Mail interacts with extensions, which causes mail to end up in the Drafts folder rather than sending when the "send" button is clicked. In my particular case, I had SpamSieve (which I have used for many, many years now) installed using the Mail extension, and was occasionally experiencing this problem. Following the discussion in the now-locked thread, I disabled the extension (SpamSieve can work without it), and since that time - now nearly two months ago - I haven't had any sent mails end up unsent in the Drafts mailbox. It's not conclusive proof yet that Mail's handling of extensions causes the problem, but the correlation between disabling an extension and the problem no longer occurring does strongly suggest that's the underlying problem.