In the details box it says: Invalid MIME content. 

I have recently updated to this outlook and this error message continues to pop up. My inbox is cleared. Due to my place of employment, messages that are sent or deleted have to be kept. How do I know what message is not syncing? I have checked the messaged with the "name" it has displayed but I can't figure it out. Someone posted to use the search box to find it, but how can I find it if I don't know what I'm searching for? Also... will the message have something on it to let me know it is not being synced? There are 4 of these messages that will not sync. Please help..

I have office for MAC OS X 10.11.2