One or more items in the folder you synchronized do not match

One or more items do not match

The error message "one or more items in the folder you synchronized do not match" indicates that Outlook has conflicting items that it cannot resolve on its own. You will need to open Outlook, find these items, and resolve them manually. You may also see the error “The custom form cannot be opened. Outlook will use an Outlook form instead.”

Microsoft has a guide covering some fixes for this issue here.

Exchange Accounts

If you use an Exchange account, the issue may be due to offline items stored in Outlook conflicting with items in the Exchange account.

1. Open the Calendar pane in Outlook, and right-click the Calendar folder.
2. Select Properties.
3. On the General tab, select Clear Offline Items.
4. Select OK.

Non-Exchange Accounts (IMAP or POP3)

First, open the area in Outlook that had the error message and perform the following steps. If the issue is within contacts, go to the contacts area, if the issue is within calendar, go to the calendar, if the issue is within tasks, go to the tasks area and so on. If you do not know which area this is, please repeat the steps for each area of Outlook (Contacts, Calendar, Tasks, etc):

1. Set Outlook to list view, this can be done by clicking the View tab (a in image) > Change view (b in image) > List
2. Find the "Categories" column and click the label to sort by categories
3. Look for the "crossed swords" icon to the left of some items, this means those items are in conflict
4. Double-click each item that has a "crossed swords" icon
5. From the Conflict Resolution area select the item you wish to keep, typically you want to keep the item with the most recent date (ie the record that was edited most recently) but this may not always be the case.
6. Repeat this for all items with this icon to resolve this issue

Other Solutions

Here are some other things you can try:

1. Run Quick Repair of Outlook. Click Here for our guide on this.
2. Verify your Outlook Folders are in Cached Exchange mode (if you are using an Exchange email). Click Here for a guide on enabling Cached Exchange mode.
3. Run a manual Send\Receive of the Outlook Folders and make sure there are no errors. Click Here for a guide on manually running a Send\Receive.