It is constantly frustrating that when I select an email address or subject text or anything shown in the header pane (not within the message view, but the bit above the message view), using Ctrl+C to copy the selected text doesn't work--you instead have to right-click and select "Copy". This runs contrary to how everything else in the UI works.
True, X11's clipboard doesn't work here, that's annoying. BTW, did you notice that X11's primary copy/paste mechanism works? Select some text in the headers pane, paste it using the mouse's third-button (commonly the middle one) somewhere else.