"UnknownIncomingEmailIntegrationError -2147220891 exception" error appears within mailbox alerts in Dynamics 365

When viewing the alerts section within a mailbox record in Dynamics 365, you see one of the following messages:

• "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 -2147220891 exception."
   
• "An internal Microsoft Dynamics 365 error occurred while synchronizing appointments, contacts, and tasks for the mailbox "<Mailbox Name>". The owner of the associated email server profile <Profile Name> has been notified. The system will try again later.
  
  Email Server Error Code: Crm.80040265.ISV code aborted the operation."

Error code 80040265 and -2147220891 indicates an IsvAborted error.

If you see the first message listed in the Symptom section, this is typically caused by a workflow or custom plugin that runs on the creation of an email record. 

If you see the second message listed in the Symptom section, this is typically caused by a workflow or custom plugin that runs on the creation of an appointment, contact, or task record. 

Check to see if you have any custom plugins or workflows that run synchronously on the create of the record type mentioned in the error (ex. email, appointment, contact, or task). If a plugin or workflow is causing an error during the creation of the record, Server-Side Synchronization will not be able to create the record successfully. The steps below can help you identify if there are any workflows or plugins in your organization that run during creation of an email. The same steps can be used for other entities like appointment if that is the record type that is failing to be created:

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.
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 wors 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.