During the creation of a Notification object, a distribution list was added to the recipients section. Within this distribution list, some recipients are users in the Automation Engine - others are not. Users who are configured in the Automation Engine are the only users receiving emails when mailing the distribution list via a Notification Object.
No error messages are noticed in the Automation Engine log files/Job Report/Execution data.
Any Automation Engine/Exchange Server configuration.
Configuration of Exchange server rights.
In Exchange Server (external to Automation Engine configuration), it may be that distribution groups by default are not able to receive messages from applications. Check with the Exchange administration group to turn that feature on for just the groups that are required.