I have to say it depends, it has to be checked properly. As troubleshooting steps you can try with a different email address to see if it works. Also, did that specific user receive his Okta activation email? Have you altered the email notification settings in the Okta console? Does he have any filters on email?
There are many potential mistakes/solutions.
My best suggestion for you would be to open up a case with us, not just creating a topic on the community page. In that way we have access to many more tools to help you solve the issue in a reasonable time. By having a case opened with us you can even benefit from the help of the Superior Technical Support if needed.