Exchange 2016:- Event ID 4999
On your Exchange 2016 server, you might be seeing a number of 4999 Events log. Event ID 4999 is quite a broad error as it can affect multiple things. In this instance we are looking at the error below:
As you can see above, it relates to OWA. You may be receiving this error because of an Outlook Web App Web part. They are no longer supported in Exchange 2016 and you may have a user or multiple users trying to use them.
The new way is to use the EWS Managed API.
From what I have seen this error does seem to recycle the OWA web pool in IIS but have not seen any users reporting OWA errors.
Hope it helps.