set minimum font size for reading mails

For reading mails, how can I set a minimum font size? Actually in the setup-menu I can select a specific font and font size, BUT theer are html-emails that seems to overwrite this setting. Especially when using a nearly high-dpi-monitor such mails get displayed in a far too small font size.
So is there a way to define a minimum font size that will not be overwritten by a CSS style or html tag in am email itself?

really - no-one has an idea?