It's my understanding that your server is hosted in France, is that correct?
Having lived in France myself and having used both GMX and WEB email adresses, I discovered at some point (this dates back at least 15 years) that for some reason, there seemed to be a blacklist either on the french or german side (or both), which completely prevented sending or receiving emails between the french provider and GMX/WEB.xx(x).
If I remember correctly, it didn't even matter which provider one used for internet (in France), since all used the same physical network (I believe France Telecom owned all the cables back then, no idea what the status is now). We never got any email delivery errors, everything just disappeared into the void. We contacted customer service for both the french and german sides but neither was willing to offer any solutions. I think we managed to get an error message confirming the blacklist suspicion somehow, not sure what we did, might even have been the customer support that confirmed it.
We just assumed these companies (France Telecom and 1&1 Internet AG/1&1 IONOS) must have had some kind of beef which never got resolved, and looks like this might still be the case. If so, there is probably not much we (you) can do about it:
- switch to a server hoster in a different country
- switch to a server hoster which uses a different internet provider (might need to be a different physical network if there even is one in France)
- accept it and update
https://www.returnofreckoning.com/join.php to state that GMX/WEB emails will not work
Sorry I can't give more detail, as I said this was more than 15 years ago, but at least it may shed some light.