Mail sends ok but does not arrive - just to one address!

I can send mail from my iMac using Apple Mail or Outlook and it has worked just fine to many users over many years with several Mac devices .

However there is now one email account (hosted by Hostpapa) where it looks like a perfect send from me but it never arrives. No error messages. It is in my sent folder. Exhaustive checks show that the emails are not being received nor are they in the recipient's spam folder.

This only happens when sending from one particular( and my default) gmail address and also from email aliases from that account. Other gmail, iCloud, yahoo etc emails all get through fine

The Hostpapa email account will, however, receive emails from the gmail address if I send them from an iPhone or iPad or from the gmail webmail interface

Something in my Apple Mail system seems to be preventing the email arriving at the other end (and just to the one email address).

I have seen a question raised on a similar topic here but no suggested solution

Any ideas anyone?

Running 2019 imac

Big Sur 11.1