Only add the email-addresses of the recipients when you're ready typing the text. It won't be sent without address.
And, indeed, I've never seen mail being sent before I pressed the Send button or the shortcut Alt-S or the shortcut Alt-F>S. Are you sure you don't do one of those by accident?
And there's no need to send partial mails. You can open the half one in your outbox, choose Reply all and edit it to its final state.
I have a laptop with a rather small keyboard. Almost every time I try to send an e-mail in Outlook Express, the e-mail will send before I've finished typing it. So I have to send the e-mail in segments, which is a big problem because then the recipient gets about three partial e-mails from me. This is very frustrating and has been going on for a very long time, so I decided to post this message to see if anyone had any suggestions.