I occasionally use Mozilla's Thunderbird e-mail software alongside Outlook. One thing that annoyed me however was the pop-up "Sending Messages" dialogue every time I sent a reply. Having searched Google I found a solution which involved creating a user.js file. This, despite sticking it in several suggested folders, didn't work.
However, there is actually an easier way to do it (at least in my version, 1.5.x). Open the program and go to Tools > Options > Advanced and click the Config Editor button. Then in the filter line search for "progress" and you should find a variable called "mailnews.show_send_progress". Right click on this and select toggle to make it false. Then close that box and click Ok on the Options window. Restart Thunderbird and it should work!