Can you enable the subject header on a Mac ?

duncauk wrote:

Like iPhone iMessage can you enable a message header?

?

in Apple's Mail.app ?

You can customize the tool bar to include the Header option...if that is what you are asking.

Customize toolbars on Mac - Apple Support

or

you are saying you have no Subject in the compose(?):

If one it talking about Messages Application on Monterey and not e-mail - AFAIK - No.