To anyone having this problem, I registered just to post the solution here
First, make sure that your complete email address is in all the input fields that ask for it.
For me, in the outgoing mail server settings, the field only contained "@gmx.com" - which obviously is not a valid email address.
Then, second, make sure your GMX domain is indeed .com. For me, it was xxx@gmx.
de and not xxx@gmx.
com.
After changing everything to dot DE (.de), it worked just fine
Happy reading