If you are receiving the following error in Outlook 2010, Outlook 2013 or higher:
Task reported error (0x80040600) : ‘Unknown Error 0x80040600’
and you are using either Microsoft Exchange or Office 365 please follow these instructions to resolve the error. Specifically the error is caused due to a corruption in your Outlook OST file.
Step 1. Close Outlook
Close outlook and any other programs which may be accessing Outlook in the background such as Skype for Business.
Step 2. Delete the OST file
Browse to the location of the OST file for your profile. On a Windows 8 machine it is most commonly located at:
Right click on the OST file and delete.
Step 3. Open Outlook
Open up Outlook again and the OST file will automatically be recreated and Outlook will be synced with the server again.
This error occurs due to corruption in your Outlook OST file and applies specifically to accounts synced with Microsoft Exchange. It does not apply to PST files, deleting a PST file will delete all your data. If you need to repair a PST file please use the SCANPST tool.