Thank you Shal and Gerald.
I have scoured the activity log and there is no sign that the messages ever arrived. I suppose some combination of local wi-fi or router being down and some failure of the mail client could account for it. Enough time has passed that the messages were evidently not held up in the maze of store and forward. The problematic messages were sent from a Mac, but I have had issues on Windows during Msft updates lately in which apps suddenly become unresponsive and entries disappear. Something similar may have? happened.
Since no harm was done, other than a tiny undeserved chink in groups.io's reputation for reliability, I will quit kicking this sleeping dog. Closed-no resolution.
Thanks again, and best, Marv
Marvin Waschke