It may be gmx.com or gmx.us which I happen to have. I know their website will link to gmx.net but no option for that gmx.net exists. It may be legacy. Hope you can figure it out.
Let me start by saying that I'm just another user of Ubuntu MATE and of these "Ubuntu MATE Community" Discussion Forums - https://ubuntu-mate.community/ - that I know are based on / powered by the forum software called "Discourse" - https://www.discourse.org/
Based on this post of yours, I 've tried to add an "Alternate Email" address in the "Email" section of my "Preferences" -> "Account" page here in the Ubuntu MATE Community. In that "Email" section, I've added a Gmail e-mail address of mine and after I've added it, a web page showed a message saying "We've sent an email to that address. Please follow the confirmation instructions.". Well, two hours have passed since then and I haven't received any e-mail message with the confirmation instructions. I've double-checked the Gmail e-mail address of mine that I wrote (it was right) and I've checked the "Spam" folder in Gmail to see if the confirmation e-mail ended up there (it hasn't).
So, I believe you're right that something is wrong, @rawgr . I've found the following related topic in the Community here ("Problem with Password Reset on this Forum" started 12 days ago by user @anon49499279 ):
I'm not sure who has the Administrative permissions to check the logs of the Ubuntu Mate Discourse installation and/or server to see what may be happening that is apparently interfering with the sending of confirmation e-mail messages related to the addition of e-mail addresses (primary and/or alternate) and password resets. I'm guessing @franksmcb , @lah7 and/or @Wimpy himself may be able to check this and so I'm mentioning their usernames here in order to ask them to investigate this situation, please.
Today (21st August 2022), 7 days after my previous post in this same topic, I've finally started receiving again some automated e-mail notifications related to these "Ubuntu Mate Community" Discussions.
Because of that, I went back to the page where I had tried to add a (Gmail) "Alternate Email " address in the "Email " section of my "Preferences " -> "Account " page here in the Ubuntu MATE Community. There, I clicked in the "resend confirmation email" link that appeared below my "Alternate Email" address (as I've done a few times during this past week, without any success). But, this time, a few minutes after I clicked on that link, I have received from "Ubuntu MATE Community <[email protected]>" with subject "[Ubuntu MATE Community] A new email address has been added" with the e-mail confirmation link.
So, I'm guessing someone that has administrative privileges in the Ubuntu Mate Discourse installation and/or server has fixed the problem that seemed to be preventing the sending of automated e-mail messages from the Ubuntu Mate Community (such as the ones related to the addition of e-mail addresses - primary and/or alternate -and password resets). Thank you, whoever you are!
@rawgr : Based on what I wrote above, I suggest that you try to go back to the "Email " section of your "Preferences " -> "Account " page here in the Ubuntu MATE Community and click on the "resend confirmation email" link near your gmx.net e-mail address to see if you get the e-mail message with the link to confirm that e-mail address. I hope this helps!