"UnknownIncomingEmailIntegrationError -2147220956 exception" appears within Server-Side Synchronization alert in Dynamics 365

A warning level alert like the following appears within a Dynamics 365 mailbox record:

"An unknown error occurred while receiving email through the mailbox "[Mailbox Name]". The owner of the associated email server profile [Profile Name] has been notified. The system will try to receive email again later.

Email Server Error Code: Exchange server returned UnknownIncomingEmailIntegrationError -2147220956 exception."

Error code -2147220956 indicates an IsvUnexpected error. This typically indicates a custom plug-in or workflow is throwing an error and causing the creation of the item to fail. 

The following steps use the example of an email record that is failing to be created. If you are seeing this error logged for another type of item such as an Appointment, Contact, or Task, the same type of steps can be applied by substituting references to "email" with the type of record you are troubleshooting.

Check to see if you have any custom plugins or workflows that run synchronously on the create of an email. If a plugin or workflow is causing an error, Server-Side Synchronization will not be able to create the email. The steps below can help you identify if there are any workflows or plugins in your organization that run during creation of an email:

Workflow

1. Within the Dynamics 365 web application, navigate to Settings and then click Processes.
2. Change the view to Activated Processes.
3. Sort on the Primary Entity column and look for any rows with Email as the primary entity and Workflow as the category.
   
   Alternatively you can use the filtering options in the grid to filter on Category = Workflow and Primary Entity = Email
4. Open each workflow you find that meets the criteria above (if any).
5. If the Start when options have the Record is created option is selected and the Run this workflow in the background (recommended) option is not selected, this workflow could potentially be the cause.
   
   NOTE: If you are troubleshooting this type of error with an Appointment, Contact, or Task, also check if the workflow is configured to run during update of that type of record. 
6. Click the Process Sessions section on the left side of the page and lookg for any failures related to the email that was not created successfully.

Plugins

1. Within the Dynamics 365 web application, navigate to Settings, Customizations, and then click Customize the System.
2. Click Sdk Message Processing Steps.
3. Sort on the Primary Object Type Code (SdkMessage Filter) column and look for any rows for the Email entity.
4. If you find any rows and the Execution Mode is Synchronous, this could potentially be interferring with the creation of the email.

If the issue reproduces consistently and it is possible to temporarily disable the workflow or plugin as a test, this can allow you to determine if the workflow or plugin is the cause.