Because your e-mail account is from one of the following providers who directly reject the sending of the confirmation e-mail:

• Hotmail: These include the addresses @hotmail.com, @hotmail.de, ...
• Live: These include the addresses @live.com, @live.de, ...
• Outlook: This includes the addresses @outlook.com, @outlook.de, ...
• MSN: This includes the addresses @msn.com, @msn.de, ...
• ...

Note: The emails are sent via our email server that is NOT used for spamming purposes. StartTLS, X509, SPF, DKIM, DMARC, DNSSEC etc. are configured correctly: https://mecsa.jrc.ec.europa.eu/en/finderRequest/2cb503d1eec700a3b0873778f86699be

Microsoft / Yahoo still reject the emails. Learn more: https://forum.kuketz-blog.de/viewtopic.php?f=36&t=5007
How secure is your email provider?
Hotmail: https://mecsa.jrc.ec.europa.eu/en/finderRequest/0947daacc0d0d551f1c29585759c3e44
Gmail: https://mecsa.jrc.ec.europa.eu/en/finderRequest/5174ff3de534d92113bc705dba550fb7
iCloud: https://mecsa.jrc.ec.europa.eu/en/finderRequest/fbb90fe5ea36dbceec9d9959e0218fdc
Yahoo: https://mecsa.jrc.ec.europa.eu/en/finderRequest/b76af589a6070e5f5cbfbc2cfe8c0530

Test your email provider: https://mecsa.jrc.ec.europa.eu/en/

Our recommendation, use mailbox.org or similar: https://mecsa.jrc.ec.europa.eu/en/finderRequest/3cada739769090a68daaa9d092661822

Please use this contact form if you want to send our support your old and new e-mail address.