For this, in Outlook, go to File menu >Manage Add-ins. This action will open ‘Outlook on the web’. There, under ‘Manage add-ins’ check the ‘Turned on’ column for the add-in in question.

If you don't want to see these messages, tell Outlook to hide them. Open the folder where you want to hide strikethrough messages, such as the Inbox folder. Go to the View tab. In the Current View ...