Okay, you are not going to believe this: I have solved this problem, but not by doing anything anyone at Comcast told me to do. (I called them again this morning, and they told me the problem was that my e-mail was configured as POP3, not IMAP. That was not accurate.) I called a friend who told me the problem sounded like a spam filer problem, and I agreed. It was the spam filter, all right, but not the one on my Outlook client. It was the one on the Comcast server. I turned it off, and the problem disappeared! Yes! I am so happy! And thanks to gmayor for offering help.