Outlook stopped syncing: the most common causes

Outlook that no longer syncs is a classic: this morning's mail is there, but nothing new has arrived since, or a sent message stays stuck in the Outbox. The good news is that the cause usually hides in a small corner. Below are the most common causes, each with a concrete fix you can try yourself.

Start with the basics

Before you dig deeper, rule out the simple causes first. These two together account for a large share of all reports:

• No internet connection: check whether other things work online, such as a website or a shared folder. No internet means no sync.
• Offline mode is on: at the bottom of Outlook you then see "Working Offline". Go to the Send/Receive tab and click "Work Offline" to switch it back off.

If it still does not work afterwards, close Outlook completely and start it again. That resolves a stalled connection surprisingly often.

Account, password and mailbox

If the basics are fine, the problem often lies with the account itself. Three causes we run into regularly:

1. Expired or changed password: if you recently changed your password (for example via webmail or through the IT administrator), Outlook keeps trying with the old one. Enter the new password when Outlook asks, or remove the old one from Windows Credential Manager.
2. Full mailbox: if your mailbox has hit its limit, nothing more can come in and syncing stalls. Free up space by clearing large attachments and old items, or have the storage limit raised.
3. Wrong account settings: if the server, port or account type (IMAP, POP or Exchange) is incorrect, Outlook does not connect properly. Check the settings against what your provider or IT administrator supplied.

When setting up mail, it pays to store these details somewhere safe. With a new PC or a profile that needs rebuilding, that saves a lot of searching.

Corrupted profile or OST file

If things keep going wrong while the webmail works perfectly, the problem usually sits locally in Outlook. The local copy of your mailbox (the OST file) can become corrupted, causing sync to falter. A new Outlook profile often fixes this: Outlook then builds a fresh local copy from the server, without you losing any mail.

This is the point where you need to be careful. With a POP account, messages sometimes exist only locally, and then you want to delete nothing until you are sure everything is also on the server. If in doubt, make a backup first or ask for help.