I fixed it! I was poking around looking at Windows Mail settings, and under
Tools, Options, Security, Digital IDs, I saw that there were three
certificates issued to my computer's name. I wasn't sure what the meaning of
this is (who issued the certificates and why?), but I took a chance and
deleted them. Now, I can send email! I suppose that somehow one of those
certificates must have been involved in the SSL port 465 transaction. But why
does it work now, with NO certificates. I went back and looked, thinking that
maybe Windows auto-generated a new certificate, but it did not. There is only
one certificate remaining, one I made myself long ago when playing around
with PGP (Pretty Good Privacy). Maybe it used that one?

Microsoft must have some kind of problem with certificates and SSL and
perhaps related to changing of passwords on email accounts.