We had the problem that the text in html-emails stays too little, even when we changed the font size in the em client settings. Then we found out: The Zoom-Shortcuts are not working, at least not under Mac OS. But it works if you use the right/secondary-click and than manually click “increase front” etc.
A big plus: em client then remembers the settings and will show all emails with the bigger font setting automatically.
(I am using em client on mac os 15.6.1 right now and I use em client version 10.3.2622.)