Using Classic Outlook on Windows for personal email, calendar, and contact management
Workaround:
Right click on the From header at top of emails. Then Field Chooser
Then New, change the Type to Formula, give the new field a name (e.g. MyFrom). Enter [From] in the Formula box.
Click OK. Now click and drag the newly created Field from the list onto the emails beside the existing From field
Now, click and drag the original From field off the email list. The EXTERNAL Label is tied to that field, and by removing the 'real' From field it will take away the label.