I've got an issue where all my SharePoint task lists have stopped sending automatic email notifications whenever a person is added to it. Why is this happening and how do i solve it?
Troubleshoot as following:
Investigate the user's permissions for the list, for the library, or for the task list. Make sure that the user account has at least Read permissions on the object. If these permissions aren't present, direct the user to the site administrator to have these permissions corrected.
If these permissions are present, create a new alert on a test library or on a test list. Perform an action to generate the alert. Then, wait 15 minutes. If the alert isn't received, collect the following information, and then contact Microsoft Office 365 technical support.
If the new alert is received but existing alerts aren't received, delete and then re-create all the user's alerts on the site.
If all alerts from multiple files or libraries are not delivered, visit the Service Health Dashboard from the Microsoft 365 admin center, Health, Service health to check for any advisories/incidents that may be occurring with SharePoint or Exchange. The issue could be with the SharePoint alert capability or delays in emails through Exchange. It will also be important to note whether other email is being delivered, and if not, the issue is likely with Exchange delays.
More information check the article: Users don't receive SharePoint Online alert notifications