Firstly, check the sent auto-responders and filter by the registrant's email. To do so go to the admin panel > Email > Sent Auto-Responders > filter the Email column by the registrant's email. You should see all the emails sent to this registrant. Click on each of the sent emails and scroll down to email stats. If the emails were sent but not delivered means they either opted out or marked it as spam. In this case, notify email@example.com and ask to remove the email from RB's suppression list.
If the emails were delivered successfully but not received, these might be a few causes:
1) If this email address is not receiving emails from other sources either.
2) If the inbox is full.
3) In terms of work email addresses, if the email server is down or having problems.
4) The email is caught on spam or it was deleted when detected as spam/hazardous.
Regardless of the above scenarios, add the sender's email in your email account > contacts list. This would help your email account to determine that emails from this sender are not spam/trash.