The FailedItems log is in the same format, for filename, location, and content as the SyncTask log with the execption that the log file name is extended to include the term ‘FailedItems’ in the name. For example: joe@source.com-to-joe@target.com.FailedItems.svclog.
This log file will only exist if one or more items fails to sync or update during the processing of the sync task. If this file does not exist, no item failures existed for the sync session.
FailedItem log entries will try to report the folder path of the item and the subject of the item. However, depending upon the error, access to the subject of the message may not always be possible.
Not all item failures are permanent. In many cases an item failure is a transient issue and may succeed upon a second attempt if the sync task is reset and restarted. Office 365 is especially common for network disconnects and timeouts which can cause item failures and which are non-permanent.
Note as well that item failures due to size limitations are also considered non-permanent. Each subsequent restart of the task will retry the items failed previously. Office 365 does have default and maximum item size limits. Reference this Microsoft article for details of such: Exchange Online Limits.
While Office 365 has prescribed limits, on-premises Exchange does not necessarily have maximum limits, but of does have default limits. Refer to the specific configuration details of the Exchange environment with regards to item size limits if there is concern.
