No column names in Apple Mail...

What exactly do you see in Mail?

Take a screenshot on your Mac - Apple Support

Use command-shift-4 on the header of the Mail window (no need to show any of the messages).

and use the mountain icon below the post to grab the screen shot off your desktop to post the header.

thedeerhunter wrote:

Thanks; that would explain it, but I would have to wonder about Apple's rationale for removing them.

You're welcome. I have no idea but since they restored them in the next version of macOS it seems they had second thoughts themselves. 😎

The column headers in Mail were removed from Catalina. If you control click next "Sort by Date" you can select a different Sort by. Column headers returned in Big Sur and later.

Glad to help.