No joy after my last post. Spent a lot more time with Apple support, and the Apple devices have been eliminated from the calculus. I returned focus to the server. The question came up more than once, "what changed?" I was thinking of software updates and installs; nothing came to mind or was in the notes. It took a close inspection of the system logs, for the day the email behavior changed, to "recall" the change. Thanks to the software gods for logs.
On the day when the behavior changed, I had deleted several old, unused user accounts and locked a couple of others. I haven't found any other forgotten changes. So, next step is to examine the server email options. I plan to run a post-upgrade, reboot event, so see that puts things back in order.
At this point, it appears the smtp authentication is not working as desired. Using telnet, I could connect to the smtp service but I cannot send to any email address outside my domain. The server responds to RCPT TO: with a 550 relay denied message.
I would much rather spend my time working on the SME 10 release. I encourage any reader so inclined to lend a hand to that effort. Many hands make light work.