I have looked at the DNS records at the actual ISP, and those are all correct in that they direct all mail to the xxx.com service, and all the rest of my email addresses are working correctly and are handled there.
The only other thing I could think of was Plesk itself. Transferring to the new server updated me to Obsidian, their Version 18 iteration. There are actually less screens and menus handling mailing matters. Those are correctly set, and the DNS records Plesk independently maintains match those of the ISP. But I think the ISP records would in any case take precedence over Plesk.
The only other thing I could think of was that I do not actually use the email services on the domains themselves, my experience being that they are all too prone to hacking. I have therefore sent emails to the domain form a gmail account and responded to it, and sent another email from a gmail account and responded to that, and each work just fine.
I do not know what else a simpleton such as myself can do by way of testing to try to isolate things, but if you have any suggestions I will certainly do what you request.
Otherwise, can only wait for someone to respond to my submitted ticket.