ErrorInternalServerError: An internal server error occurred. The operation failed., Error in fanout. MessageText: The specified object was not found in the store. The process failed to get the correct properties.

In the sync logs you see the following Fanout error message:

ErrorInternalServerError: An internal server error occurred. The operation failed., Error in fanout. MessageText: The specified object was not found in the store., The process failed to get the correct properties.

This is due to a configuration issue on the exchange end. A few suggestions that have assisted the mail administrators in the past:

• If this is a hybrid environment ensure the mailbox is on O365 not on prem
• Where possible try and move the mailbox to another mailstore
• Is this a shared mailbox - see 
• If it's a new user ensure it has a mailbox with something in it

We are not 100% sure of how to resolve this and would appreciate hearing back from the mail administrator as to what worked for them.