Symptoms

When reviewing Emails, some Emails have a status 'SENT' but the recipients never received the Emails.

Cause

It turns out that the notification was processed but behind the scene, some business rule prevented the Email from being sent based on the whitelisting conditions created on business rules.

The whitelist list of Emails is typically stored in a system property named 'glide.email.test.group.custom'
The business rule name is often ''Email Test Group''

Some business rule customisations are often added to Servicenow instances and administrators are not aware of its existence.

This business rule adds a user whitelist functionality that lets an admin specify which users can or cannot be used as a recipient. The users can still be added but the Email does not get sent.

Resolution

Disable the customised business rules tampering with the notification processes (e.g. "Email Test Group")

Note: This is a customisation, you need to contact the developer to fully understand the scope of this business rule and its configuration.