I'm using a high-DPI screen. In System Settings → Fonts, I set "Force fonts DPI" to a larger value (currently 192), and most application text gets increased in size. Not so with emails rendered in KMail2 (I can open the email and zoom with Ctrl+WheelUp, but it's not automatic). FYI Konqueror seems to have the same problem. So does Firefox, but it has its own workarounds (and it's not KDE software). I found two maybe-related bug reports; both about print size: https://bugs.kde.org/show_bug.cgi?id=372662 https://bugs.kde.org/show_bug.cgi?id=78264 Alternatively I guess it would be okay to scale email rendering by the screen DPI, not by the "force font DPI" setting. As a workaround, setting the env var QT_SCALE_FACTOR=2 would "work" — but it's an ugly hack which doubles the size of everything, including the useless margins and all the UI elements which have already been increased in size (through System Settings → Icons, Application Style, "force font DPI", and possibly more); i.e. to use it effectively I'd have to set it desktop-wide at startup and revert all the other tweaks, then put up with huge margins in all the UIs. Summary: automatically scale email rendering by "force font DPI" / default=96, or by screen DPI
Further to this — received emails are displayed with very small fonts, as are emails composed with plain text. Emails composed with "rich text" enabled — *also* display with small fonts, but this isn't because of the display scaling — the font size defaults to 6pt! (I only realised recently when a recipient complained about the small font size.)
BTW, Konqueror has a 'Zoom to DPI' setting in its View menu, which works nicely for me.
Still present in kmail 5.9.3 Akregator has the same problem with the html view showing everything really small when opening an article in a tab instead of external browser So this seems to be a more general problem where the embedded html view component (whatever that is called) is not respecting the dpi setting