If your Outlook is using the simplified, or single line, ribbon you may find the Mailock button has disappeared, even though the Mailock tab is still showing, as demonstrated in the image below (if you don't have the Mailock tab either, the add-in may be disabled, please take a look at this article instead).
This situation can happen in your main Outlook window, or when writing a new email, as Outlook decides which buttons to show on the ribbon. If there’s not enough room, then some are moved to an ‘overflow’ menu. You can see it in action by re-sizing the window. As you make the window smaller, buttons are taken off the ribbon and added to the overflow menu. Similarly, as you make it larger, buttons are added to the ribbon and removed from the menu.
You can see the overflow menu by clicking on the ‘…’ button on the right of the ribbon:
This then shows the Mailock button, and it works the same as if it was on the main ribbon.
If you want to show the button on the main ribbon, just click on the ‘…’ button, right-click on the Mailock option and choose ‘Pin to Ribbon’:
The Mailock button appears as you’re used to, on the left of the ribbon:
If you change your mind and want to put the Mailock button back on the ‘…’ menu, right-click the Mailock button on the ribbon and click on ‘Pin to Ribbon’:
