There maybe times when you see an email in the "Sent" Mailbox on your instance but you don't know where it originated from. Following the procedure given below will help in such a case.
- Open the sent sys_email record.
- Scroll down to the 'Headers' field.
- Look for "X-ServiceNow-Source:".
- If the email originated from a Notification, it will show "Notification-<SYS_ID_OF_NOTIFICATION>".
- If the email originated from the Email Client, it will show "EmailClient".
This is applicable to all releases of ServiceNow.