You surprise me. I use right-click for contextual action on the specific object I right-clicked. For a Select All I use ctrl + A, or a menu button.

For me, right-click is already a selection, already a pointing at, which is contradictory with Select All.

Even in MS Windows' desktop, it would surprise me to see a Select All Icons option when I right-click on a specific icon.

When right-clicking on a specific message, a Select All (Ctrl + A) option appears.

It seems to me contradictory with the user's intention of selecting a specific message. A Select All option in a checkboxes column, would seem more appropriate.

There is also of course the Ctrl A, for the computer-literate.

I, and I'm sure many other people, open that right-click menu when looking for general options related to the whole pane. It is logical to right-click in that pane when you want to select all the messages in that pane. The menu appears whether you right-click on a message or an empty part of the pane. I don't see an issue here.