I see why you have a rule against throwaway email addresses, and I agree with it. I used to run a small forum and did the same thing. But what I didn’t then do was block all email addresses from a certain email provider, because the best case scenario then is that the user breaks the “no throwaway emails” rule, and the worst case scenario is that they just leave the site.
I’m also curious what problems there are that you think is exclusive to GMX.
So, you’ll be changing your e-mail address here to something a bit more permanent so that you comply with the Registration Agreement, right?
We don’t ban entire domains because of one user, or even a few users. We have to have persistent problems from a domain before it ends up on the ban list.
GMX has legitimate users, but it’s also a very well known source of spam. It has (or had, some of this info might be a bit out of date) a reputation for ignoring spam reports, which makes it an attractive hosting site for spammers. It’s also had problems with hackers getting into their user info and using otherwise legitimate accounts to send out lots and lots of spam. Thousands of accounts were compromised back in 2012. It’s pretty easy to see how it ended up on the ban list.