^
I see that My Account --> Overview --> Manage subscriptions does indeed work, though in my case it lists exactly the same posts as My Account > Overview > Show Your Posts does, at least for the first 20 or so posts, I didn't look further down than that. But both are good.
GMX advised me that to register a sender against their spam filtering antics I should add them to my "contacts list," so I just took a look and [email protected] is definitely there. Not a pleasant experience, using their web-based email access, Thunderbird is much better, but with that I can't access their contacts list, so I had to bite the bullet. Anyway, unless GMX is doing something really stupid (wouldn't put it past them!), the notifications simply aren't being sent.
Ironically I opened that GMX account so I could get away from Microsoft, but it's going the same way. When I tried to open a second GMX account recently, they demanded a mobile phone number, so I went to Proton. They're almost as bad, and even worse in some ways, but at least no mobile phone demand - yet.