message failed to send and cannot be deleted
I am stuck with a message in mail that will not send, and cannot be deleted. It has an overlay of a message saying 'the server 'stmp.gmail.com' failed to recognise the following recipients...... How can I delete it? The red 'close' button is absent, and none of the options work. I have tried to move it to 'trash' but it does not move.
MacBook Pro (Retina, 13-inch,Early 2015), OS Sierra