This was quite an interesting error for a mailbox I tried to migrate from Exchange 2013 to Exchange Online. The mailbox synced to 95% and I received the below error after I tried completing the batch. Error details: The formatter threw an exception while trying to deserialize the message: There was an error while trying to deserialize parameter http://tempuri.org/:ISourceFolder_EnumerateChanges2Result. The InnerException message was ‘Maximum number of items that can be serialized or deserialized in an object graph is ‘2000000’. The key to this error is the object graph limit that is set in the MsExchangeMailboxReplication.exe config file. To resolve the issue do the following: Logon to your on premises Exchange…