This alert is reported when the instance get a lot of emails to process and it takes some time to process, and hence appear stuck. Sometimes, the inactivity of the reader can also cause this.
The Email Reader jobs can sometimes retrieve a lot of emails at the same time, which can cause the event queue to be overloaded, so it takes some time for the incoming processes (e.g. inbound actions) to clear out the pending actions. So, at this time, they appear to be stuck, but they could be functioning. Sometimes it can also happen that the Email Readers are not active.
There are different causes for the alert. Here is a list of possible causes:
Please ensure to validate any change on your non-production instance before making changes to production.