catch 22.. text size: mobile-first or desktop-first?

I have what seems like a Catch 22 situation, unless I've missed something

Gmail app on android and Outlook on desktop don't support media queries (as well as a few web clients).

the desktop design I've been supplied uses a body font of 14pt bold.  this comes out too large on mobile. so I need a smaller font

If I build mobile-first with the smaller font and override for desktop in media query , then it'll be too small in outlook etc... since media query isn't supported.

if I build desktop-first with the larger font and override for mobile in media query, then it'll be too large in Android Gmail app ...since media  query isn't supported.

only option seems to be to start with an in-between font size (font-size 11pt, line-height 15pt) for devices with no media-query support.. but this doesn't work with our layout properly (there are some borders left/right and the text should flow roughly to match the height)

any suggestions please?  I know I can do Outlook-specific rules, but I don't think that's the only desktop client without media query support

thanks
J

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