How to stop Mail sending a message?

I start sending a message in Mail, then realize the attachment is too big; I'd like to reduce its size. So I want to stop the sending. I think there used to be a way to do this, but I can't find it. I open the Activity window, but see no way to stop the process. Maybe it's because I'm using an IMAP service? But what's taking time is uploading the message to that service, so it should be stoppable before it's finished. Is there a way to stop the sending of a message that's in process?