Using Backspace to delete list when the delete key won't

[pianojoe]

I like to share worthwhile email with friends. When I click "Forward", there is often a lengthy list of previous recipients. Click/dragging, then hitting the delete key does not delete the selected list. I have to use the Backspace key, which does delete what I want, but it is a one-line-at-a-time, slow process. Any ideas why the delete key won't work with this scenario?