I have a friend from the UK who – like me – uses a GMX mail account or two. The only difference is that her domain is gmx.com while I’m using gmx.net.
And twice already some mail of hers landed in the internal gmx “assumed spam” folders which you only see via their web interface if you look – so my advice: don’t ignore their regular messages about that assumed spam folder, and go and check it – you might have missed someone who clearly didn’t send spam (in my friend’s case, not even a link was inside her mails, so it’s not clear why the gmx algorithms assumed that to be spam…)
I’ve told their web interface to not classify her mail addresses as senders of spam, still have to check if that works.
Otherwise they’re very good – and they really keep out lots of spam. Not bad for a cost-free provider at all – and if you’re using them with an email program like I do, you won’t even see all the ads which they need to finance it all.
Just a hint to go and check…