How can I stop Outlook Express in the process of sending an email?

You can remove the Ethernet cable from the machine. Outlook Express will time out and won't send the message. On a system connected via Wi-Fi, you can turn off the "radio" and achieve the same result. In some cases, you can hit <CTRL><ALT><DEL> and terminate Outlook Express ungracefully.

I don't recommend any of these methods -- it's for information only.

Thanks. Good information and useful.