My mail server was not delivering the activation mail due to the following error:
Oct 09 02:05:17 [postfix/smtpd] NOQUEUE: reject: RCPT from so254-55.mailgun.net[198.61.254.55]: 450 4.1.8 bounce+782af7.904e8-stefan=abcdefg.de@mg.theta360.guide: Sender address rejected: Domain not found; from=bounce+782af7.904e8-stefan=abcdefg.de@mg.theta360.guide to=stefan@abcdefg.de proto=ESMTP helo=<so254-55.mailgun.net>
One might want to fix that. I disabled temporary the “reject_unknown_sender_domain” in postfix to receive the mail.