Have you recently deployed a new mail flow rule in Exchange Online that gives a user the option to override a warning and send the message? A common scenario is that the policy tip does not show in the desktop version of Outlook but shows immediately in Outlook Web App.
The solution is pretty simple. Supported versions of Outlook only check for new or amended policy tips once a day. To force it to check again you need to delete a particular registry key.
The example here illustrates the registry key for Outlook 2016: