So if you had SharePoint 2013 alerts configured and working and is just stopped working one day with the following symptoms:
- If you register for alerts in a particular list, you get a notification that "You have successfully created an alert for..."
- If you modify any item in the same list as above, no alerts come thru.
You will need to make sure that each SharePoint server ip address (web front, application) in the farm is added as an allowed SMTP sender in Exchange.
Explanation: Any SharePoint server in the farm can be in charge of sending an alert and usually one is in charge of a particular site collection. If you restart Timer Service, it might move that responsibility to another SharePoint server whose IP is not listed as an allowed SMTP sender in Exchange, and as an end result, it might not send alerts.
Hoping this helps someone
