Using Classic Outlook on Windows for personal email, calendar, and contact management
Great tip, which our IT department passed on to me, thanks for this. I have also discovered something which may help people: I added the new field as suggested here and realised I couldn't sort on it (something I do regularly with my overfull email inbox....). So I brought back the original From field and by chance put it to the right of the new field I'd just created --> which made me realise that having a field to the left of the From field suppresses the external label....!
So I'm now keeping my newly-made column to the left of the From column (but having it about 3 mm wide in the display) and using the original From field (and its ability to sort) but without the very, very irritating external label.
Hope this helps someone else too!