Apple Mail font not displaying

Over the last few years when I have upgraded OS on my 2017 iMac (and on previous Macs) fonts have frequently not displayed properly and I get loads of question marks in black boxes where letters should be in received mail (see image). I can only resolve it (though not always) by quitting Mail and re-booting it.

For many years I have never managed to resolve this issue and it usually settles down after a few months, probably because of an update somewhere but recently I have had this issue persistently since installing Big Sur and would welcome some help to resolve it. the issue has also affected Safari in the past to the point where I had to switch to Chrome several years ago.

I am a 20+ years graphic designer using Adobe Creative Suite and Extensis Suitcase, all latest versions. I assume there is a font clash or something but have no idea how to resolve it. Any help would be most gratefully received! Thanks.