A notification object's report shows the message 'U00050006 SMTP server 'xxx' has rejected the Email Command: Service not active.'
It is then followed by notification failures.
U00011078 Changing Notification status of user '<USER.OBJECT>'. ('Called' -> 'Notification failed')
Release : 12.3.4
This message is a response from Automic sending out the SMTP protocol to the SMTP server. Reaching out to the admin in charge of the SMTP server.