The problem does not appear to be exclusively on the recipient's end.
Several people to whom I have sent emails have reported the problem.
However, I think I have stumbled on a solution. Following the encouragement
of others in this discussion, I tried shutting down the elements within
Norton AntiVirus that work with email. Upon changing the status of the
checkboxes from "On" to "Off", I then experimented with email to see if the
problem was corrected. It was not. I then tried switching those elements
off and, while they were off, I shut down and restarted my computer. After I
did that, the problem disappeared. The problem remains absent even after
switching back on all of the Norton Internet Security components. So, an
inelegant solution, but it appears to have worked. Good luck to anyone else
who is struggling with this annoyance. By the way, Symantec is nearly
clueless about this problem.