Open the Mailbox Cleanup tool. Its View Mailbox Size feature shows you the size of all of the folders in Outlook. Scroll down towards the bottom of the list to find the Sync Issues folders.


You won’t see these folders by default in the navigation pane in Outlook, so we’ll start by making them visible. At the bottom of the navigation pane click on the ellipsis (the three dots) and select the “Folders” option.



This changes the view in the Navigation pane to show all of the mail folders in your mailbox, including the Sync Issues folder and its three sub-folders.


Messages in the Sync folders are only visible within the Outlook client on your machine, so you won’t find them in the Outlook web app.

What are the Sync Folders For?

Generally, the Sync folders hold messages telling you about problems encountered when syncing emails in Outlook with those on the email server (or vice versa). This usually happens when you’ve been working offline, and Outlook hasn’t been able to connect to the mail server, or you’ve had some kind of connectivity problem.

Each folder holds emails about a specific kind of problem:

  • Sync Issues: Outlook synchronizes with your mail server on a regular basis in the background. This is why you can send a mail from Outlook on your computer and still see the message in the Sent Items folder when you check Outlook in the O365 online portal. The Sync Issues folder holds the logs for the synchronizations, so any general problems or delays in syncing with the mail server are recorded here.
  • Conflicts: If Outlook has had a problem syncing with the mail server, you may have different copies of the same email. This commonly happens when you use Outlook in more than one place, like your laptop at home and your phone when you’re on the go. The Conflicts folder holds any different copies and allows you to choose which you want to keep. If you’ve made a change to an item (like a mail, a task, or a calendar event) on your phone and it’s not showing up in Outlook, it’s probably here.
  • Local Failures: When Outlook can’t send something to the mail server, it designates it as a failure and puts a copy into the Local Failures folder. This might be a mail, a task update, a calendar response, or anything else that syncs with the mail server. Outlook will keep trying to sync, so just because an item is in this folder, it doesn’t mean that it wasn’t successfully sent to the server eventually—just that Outlook struggled to send it for a while.
  • Server Failures: This is the converse of the Local Failures folder. Where Local Folders contains things that Outlook can’t synchronise to the mail server, Server Failures contains things that Outlook couldn’t synchronise from the mail server. The Local Failure items are visible at all times, but the Server Failure items are only visible when you have an active connection to the mail server (because they’re messages from the server). Server Failure messages are much rarer, which is why this folder often doesn’t show up in the Mailbox Cleanup > Folder Sizes window.  For example, the account we’re using to show you how this works has been running through Outlook for over three years, and Server Failures folder has never had a single message in it.

Can You (and Should You) Delete These Messages?

Yes, you can delete these message just like any normal message in Outlook. Whether you should delete them is a trickier question.


These are messages that most Outlook users are unaware of because they come from the Exchange Server and are used by administrators to troubleshoot issues. Most of the time these messages are of no use to you, unless you’re troubleshooting an Exchange issue. If you’ve never done that and you need the space, then you can probably delete them without a problem. That said, these messages tend to be very small, all-text messages and you might not gain a lot of space by deleting them.

Another important caveat: If you’re on a computer provided by your workplace, be sure to check with your IT support people before deleting these messages. You might not need them, but they might.