Why does the sender not receive a bulk emailing they originate?
I haven't found the cause, but I've found a fix that seems to work.
If I call up Edit on a contact and then Save it, even without making any changes, I am able to send that contact bulk email. I cannot be sure, but I suspect that the defective contact records were ones that were imported via CSV from another platform with an older CiviCRM release.
Yes, this seems mysterious and it doesn't exactly increase my confidence in CiviCRM. On the other hand, I'm a release or two behind on WordPress and CiviCRM, so maybe it's already been fixed.
So all I have to do now is Edit/Save all 1024 (coincidentally, I think, 2**10) contacts in my database. Perhaps I'll make this a group project for my organization! B-)