Ok, I found the problem after some trial and error. I was trying to start the sendmail program with a -d21.1 option to see what was going on, when I noticed that the daemon instance of sendmail was not restarted by the startup-items "Sendmail restart" command that is in the "update" script outlined in this article.
It took a full system restart of my machine, and now mail is working properly (one of the downsides to having a stable operating system!)!
I hope anyone else caught in this issue can benefit from my learning process!