I have the similar problem of inadvertently triggering the sending of a message while I am editing it. It happens when I am typing fast and I don't know which sequence of keys causes it. (I don't have any system hotkeys set in Options / Other options / System Hot Keys.) One way to send a message is Ctrl + Enter. I couldn't disable this combination of keys in The Bat!, so I sent up a system hotkey for all Windows applications in Directory Opus, software that I always have running. I set its function to Properties, causing a benign error when it happens. This has trapped and prevented some unintentional sending, but unwanted sending still occurs sometimes and I don't know why.
I do have Confirm Immediate Sending enabled (checked) for each mail account in Properties / Options. This also catches some mistaken sending, but sometimes I strike a sequence of keys that confirms the sending, which I didn't intend to initiate to begin with.