Thanks for all the suggestions; I appreciate them. Still no luck.
- Yes, it's spelled properly: smtp not stmp. (Good catch on the typo)
- No, authentication is not enabled. (When I enable it, I can't receive or send. When I disable it, I can receive, but not send.)
I'm still intrigued about the possibility of the messages being "stuck" because they are marked as read. One piece of advice, found elsewhere, suggested that Microsoft OneNote can, under some circumstances, automatically 'read' the mail in your outbox and cause this problem. I uninstalled OneNote and Outlook, then reinstalled only Outlook to see if this would make a difference. Same results.
Another suggestion was to ensure that the outbox is not selected, as showing its contents on the screen might also cause a 'read' to occur. Did that too, again, no luck.
Very frustrating. I'm getting by by using Compuserve's web browser for sending, but I'd rather stay in Outlook.