Emails sending instantly with keystroke by accident! What keyboard command does this?!?

Using my comcast/xfinity email account. My mouse is nowhere near the send button when this happens, and I am typing when it happens, so I am certain it is a keyboard shortcut I am triggering my accident. The key, or key combo, is somewhere on left side of my keyboard. This has happened twice, but I can't seem to duplicate it. There is some button over there, or combo, that instantly sends my email. So, out goes a half-written email! No message of any kind precedes the send...whatever keystroke it is just acts immediately. Help! Does anyone know what this could be??