We are having this issue since at least this morning and for all 3 environments.
This situation was addressed through another support channel; providing a summary here for the possible benefit of others.
When dealing with issues associated with email deliveries take in consideration if you’re using your own email provider or the default test provider.
If using the default test provider then occasional delays in email deliveries are somewhat expected as this is nothing more than a test provider which is not meant for production use.
If using your own email provider please ensure that the issue is not downstream to Auth0, for example, it can be a problem with the email provider itself. A quick way to troubleshoot this situation is to configure a non-production account with a custom email provider pointing to a mailtrap account and see if the emails get delivered there. Although not a definitive test, this can help pinpoint the root cause to either the other email provider or to how Auth0 integrates with the other email provider.
In this case, the situation ended up being an issue with the email provider itself.