I have never known about this feature. Arnaud is right - it's awesome. I wish it was documented somewhere. If it's so hard to miss then it might certainly be worth adding to the right-click menu.

I wonder how many other undocumented tricks there are that I've never found just because I use the program in a very straightforward, "proper" way?

One of the features I find great in Pmail is the possibility to start a Reply message quoting selected text only.

Maybe this could be enhanced by a right-click equivalent action.

Of course the form that appears should also have the first line "Include..." already checked.