Quickly google "gmx account blocked" and see for yourself the countless people who have had this issue.
Been going for years and still not fixed.
They are unreliable and systematically block people's accounts for no reason. Often have to wait days, weeks, occasionally months for them to sort it IF they even sort it at all.
Customer support either goes silent or never gives you a straight answer as to why.
AVOID GMX if you value your email.