If Outlook on Windows desktop is running in Cached Exchange Mode, you may see the folder called Sync Issues in the list of folders. This folder contains the logs about all problems in synchronization of the local cache file (.ost) with the Exchange Server. 

In most cases the errors in the Sync Issues folders do not mean you have issues with mailbox synchronization and it is all right to ignore them.

If you notice message delays and/or some messages stuck in Outbox together with list of errors in the Sync Issues folder, your local cache file may be corrupted. You may need to repair or rebuild the .ost file

Read the Knowledge Base article on Improving Outlook Performance for more information on how to prevent corruption. 

Sync Issues

Sync Issues folder is container for all of the synchronization logs. The data in the Sync Issues folder is local and not copied to the server, you cannot view this information from any other computer or from OWA.

These synchronization errors occur when a mailbox item was modified in a second location before the Outlook had synchronized local changes back up to the server.

This usually occurs when a mailbox item was delivered to the desktop Outlook client and a mobile device, changes happened in both locations and the second change had been synced before the first change was saved to the server.

Below is short explanation on what is located in Sync Issues subfolders. For more details on every type of log, read Microsoft Article

Conflicts

Contains all the multiple copies of conflicting items in your mailbox. If you have made changes to an item, but are not seeing the updated item in your Navigation Pane folders, check for an InfoBar notice at the top of the item. It will bring up a list of conflicting items and allow you to resolve the conflict by determining which item you want to keep.

Local Failures

Contains all items that could not be sent to your server. You can only see these items when you are offline or using Cached Exchange Mode and you cannot view them from any other computer. Items may appear in this folder if they contain attachment incompetibl with the server. 

Server Failures

Contains items that Outlook failed to synchronize from your Exchange mailbox. The contents of this folder are available only when you have a connection to your server. Items appear in this folder only when they ccan't be written in to a cache .ost file.

Modification Resolution

User may receive the Modification Resolution email to his Inbox when he sends an email message by using Outlook with Cached Exchange Mode enabled.  It occurs shortly afterward, when an error is logged in the Sync Issues folder.

To stop the Modification Resolution logs to be sent to user, configure a registry key:

  1. In Windows click Start > Run and type in "regedit".
  2. Navigate to HKEY_CURRENT_USER\ Software\Microsoft\Office\14.0\Outlook\Options
  3. Open the DWORD  “EnableConflictLogging”
  4. Change the value.
    0 = Never save Modification Resolution logs
    1 = Always save Modification Resolution logs
    2 = Save Modification Resolution logs when a "critical conflict" occurs