I have no evidence that the confirmation email is not being sent by Nabble. I've had one myself within the last half hour.
In virtually all cases non-arrival of the address confirmation email means it has been treated as spam by the user's email service.
Very rarely - I can only recall it happening once in the nine years I've used Nabble - Nabble has been blocked by an individual server on the grounds that it is sending out too much spam. That cleared within 24 hours.
In your case I guess it is also possible that an ISP is filtering out mail that includes terms like "Adult" and "18 plus".
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.