How do I add or remove columns from the message list display?

Only for illustration purposes and to build on previous posting

Control Click Header or Right Click Header brings up a Sub Menu

Choose what you want from the available Options presented

Let me start by a Welcome and Thank you too.

To be honest, it took me some time to discover the suggestion provided early.

It was only through observations from More Learned Contributors like @Barney-15E and others suggestions did I too discover this solution.

Not ALL UI interface functions are documented in the True Sense of documentation by Apple.

That is why, we have these Apple Hosted Support Forums to ask questions and get individualized suggestion.

Barcelona-Babe wrote:

Thanks. It’s hardly intuitive and NOT covered in the text available under “Mail Help”. Have we given up on clearly documenting the human interfaces to apps in the apps themselves? It certainly feels like it….

It’s been a standard in macOS since OSX 10.0. Might have predated that.

That’s how lists work.

As with all lists, ctrl-click on the header row to add or remove columns.