This warning/notification appears generally when replying to or forwarding an email message with or without an attachment. For some, it has appeared with new messages too.
Reason: In this case, the cause relates to an 'Add-in' from Adobe and disabling this can provide a cure.
Open Outlook and select File > Options > Add-ins and select Go (see below):
The ‘COM Add-ins’ options box (see below) opens and in this, find and uncheck the ‘Adobe Document Cloud Outlook add-in’. Select OK and then close/re-open Outlook and test with an email that has an attachment in particular
NOTE: as I don’t have Acrobat installed, the add-in does not show and also, it may require other Adobe add-ins to be unchecked should the problem persist.