I use a third party app called Better Touch Tool and configured it such that it replicates the pinch to zoom gesture for mac mail in a manner that's just fine with me. On another forum, someone else mentioned the same solution but that each 'pinch' was the equivalent of only one CMD + / - keyboard shortcut, causing repeated pinching and zooming rather than one smooth one. Since I can no longer reply there, I thought I'd mention I found a solution that works for me (on this recent and answerable forum) by adding 2 additional CMD +/- 'actions' in BTT.
So, when I pinch out for example, what's triggered is 3 immediate "cmd +/cmd+/cmd+" creating a good enough outcome for me (only 1 or 2 pinches in or out). I'll try adding a 4th (or more) "cmd +/-" 'actions' and perhaps that'll get it all in one pinch but I'm happy so far. Not as smooth as native would have been but it's good enough for me.
That other user using BTT (where I can no longer reply) was only triggering one "cmd +" creating the need for repeated pinching in or out - frustrating. Hopefully they've found a solution or perhaps find this note since I can no longer reply there.
Hope this helps someone reading.
I have no affiliation with Better Touch Tool. Just a user who finds it incredibly useful.
Regards.
The other thread: https://discussions.apple.com/thread/252997884?login=true
Better Touch Tool: https://folivora.ai