You have enabled company owned smtp server (Office365) as a new account in ServiceNow. Test connection gives success result, but emails are not sent.
When a new notification is created, the email log shows the system attempting to send the email multiple times with message send-retry.
In addition, the system Log file shows: SMTP Server returned error code 451. Email will be retried later.
Once possible cause is that the new SMTP email account is not set to use STARTTLS in the 'Connection Security' property, which is required by the email server.
Edit the email account record and checkup STARTTLS in the 'Connection Security' property. Then save the changes.