Please try the following steps:
1. Ensure that the registration email address is correct.
2. Check any junk or spam folders in your inbox
3. Set up a whitelist of e-mail addresses and add the following as secure senders: firstname.lastname@example.org.
4. Check if the recipient's mail server is able to receive and send mails properly.
5. Choose commonly used mailboxes such as Gmail, Outlook, etc.