Outlook 2019 / Microsoft 365 - Outlook Sync Stopped Functioning

Sometimes the PST and OST files get corrupt, and Outlook Sync stops working. This guide shows you how to reset Outlook 2019 / Microsoft 365.

Before you start

Outlook stores data in .PST and .OST files with a maximum size of 50GB. Make sure your computer has enough disk space, at least 50GB.

Performance drops at 12GB or about 12,000 messages in a single folder. Keep your Inbox clean by creating folders and moving messages you do not need into them. Outlook runs best with an Inbox of 4GB or less and fewer than about 1,000 messages.

Part 1: Remove the Outlook Sync entries in webmail

Step 1: Close Outlook

Close Outlook.

Step 2: Open EWS in webmail

In Webmail, go to Settings, then Connectivity, then open EWS.

Step 3: Delete the entry

Click Delete.

Step 4: Confirm

Click Delete again to confirm.

Step 5: Open Outlook (MAPI)

Open Outlook (MAPI).

Step 6: Delete the entry

Click Delete.

Step 7: Confirm

Click Delete again to confirm.

Part 2: Remove your Outlook profile

Step 8: Open Mail in the Windows Control Panel

Open the Windows Control Panel. In the drop-down at the top right, select View by: Large icons. Then click Mail.

Step 9: Show profiles

Click Show Profiles...

Step 10: Remove the profile

Click Remove.

Step 11: Confirm

Click Yes.

Part 3: Create a new profile

Step 12: Add a profile

Click Add.

Step 13: Name the profile

Name the profile EnGuard and click OK.

Step 14: Enter your details

Fill in the following, then click Next.

A) Your name. Recipients see this on all outgoing messages.
B) Your email address
C) Your password, typed twice

Step 15: Finish

Click Finish.

Step 16: Click OK

Click OK.

Step 17: Let Outlook sync

Open Outlook and wait for your folders to update. Leave it open until it only says All folders are up to date. This can take a few hours, depending on how much email you have.