Hi again, @rawgr
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!