Mouse copy&paste (pasting, to be exact, copying by selecting with the mouse, works) doesn’t work in the email app in the “To” field. It does work in the subject field and in the body field; it’s only in the “To” field where you can’t paste an email address just clicking the middle button of the mouse, a very simple and handy method that many Linux desktop users are accustomed to use.
“Normal” copypaste method with Ctrl + c and Ctrl + v works well, though.
This is on Firefox 56. Haven’t tried other browser, but since it works in any other message fields I suppose it’s not related with browser.