It came to me about a minute after I posted the message. The "attempt to deliver" messages are being generated by virus mail that is using my e-mail address as a "spoofed" return address. The stupid automatic systems don't know the return address is spoofed, and generate the information message to my address.

Some poor schlob with my e-mail address in their files out there somewhere is both sending me viruses, and sending them to other people with my return address. Rats. Naturally, I have no idea who this could be. Between personal contacts and commercial contacts, I would venture my e-mail address is lodged in several hundred computer systems out there. The fact that I have not yet received any spoofed return addresses I recognize would argue for a commercial computer rather than one belonging to a friend.