Yes, same here - GMX works again with MMM :-)
PopTrayU comes with libeay32.dll and ssleay32.dll V1.0.2d from 2005, so the SSL version does not seem to be the problem.
Same problem here with GMX. Tried http://grigsoft.com/mmm-297.zip with higher TLS support but still have the same problem. I also tried https://www.stunnel.org but could not get it work with MMM and GMX. https://sourceforge.net/projects/poptrayu works but after 20 years I would like to stick with MMM.