Steps to recover
1. Click on FILE tab (Outlook main screen, Top Left Corner)
2. Click on Options (left side corner of the window, on the blue strip, 2nd last option)
3. On the window which pops-up titled– "Outlook Options”, click on Add-Ins (left side corner of the window, 2nd last option)
4. Scroll to the bottom of the list and highlight the Add-in you want to enable under 'Disabled Application Add-ins'.
Underneath that, you will see the following:
5. Change the drop-down menu to 'Disabled Items', click "Go".
6. You'll see a list of Disabled Add-ins. Click on the Add-in which has to be activated
7. Click 'Enable'.
You may need to completely close out of Outlook and go back in before the Add-in will reappear.
Note: In the case the Outlook Add-in 'crashes' Outlook and the computer admin is unable to enable the add-in again within 'Manage Disabled Slow Add-ins".
Verify if the user is utilizing Outlook 2016, if yes:
2. Email file to user
3. Download/Run the attached file 'emailSync.reg' on their computer
Note: This file will over write any disabled add-in and should only be used as last resort as updating the user's Outlook Registry can cause issues in the future potentially.