Thanks for the info Bill. The messages are sent from the UK (various places) to an address I have set up at
https://domain-dns.com/ that forwards to my GMail address, and an additional address at Yahoo.co.uk.
This setup has been flaky for a long time. The forwards to the Yahoo address are now failing one hundred percent of the time. Messages sent to my GMail address always get through.
My apologies: this is not a Namecheap thing. I have two or three domain names that I use with forwarding. I need to stop doing that.