Tiny characters / any way to use pt/em instead of px?

We are suffering from a problem for a long time with WebTop:

One but important business partner use some emailing gateway company-wide that modifies the emails slightly for some reason (security?).

Our 13px text ends up as " <font size=3D1 " in the header info, which is the tinyest possible (size=1 in element inspector).

In the reply their own text is “3D2” which looks normal, the same as our 13px should be.

In our signature I have set 10.0pt that is the equivalent to 13px, or at least it should be, now I get “3D3” there, which is larger than normal.

A nightmare. And we are in daily contact.

It looks like something like this (on both their and our screen), in order: their signature, our message header, message and signature (cut only the first few pixels):


First I thought it’s the header, and the text inherit that, but not, each line has font size set (overwritten).

There were no problem in Thunderbird and mobile app (as I see the mobile app sets no font size at all).

Any idea? Have anyone else seen similar?

Is there any way to use pt/em in normal text as default?
Alternatively do not use any formatting (possibly not the best solution)?
Or to modify the HTML template globally?
I know there is no standard for email formatting, but what are the most used ways? Are we following them? Maybe a review could help?

This is the reason I am thinking about moving to Roundcube (and mailto: ), doing so while translating the new WebTop version, that brings nice features again as I see…

Thank you for your input in advance!