If your company has MS Outlook Webmail, by which you can check corporate email on the Internet, you might notice that when you try to reply or forward an open message, it automatically closes when you click on reply or forward. Normally, a new window would open that would allow you to add your message etc., before you send.
Upon experimentation, I realized that this only happens when Block PopUP Windows is selected under the Safari menu. This is probably because Outlook Webmail uses an 'onclose' javascript command when you hit reply, to spawn another window, (which is recognized as a pop-up window by Safari). To reply safely in Outlook Webmail, disable the feature temporarily.
Mac OS X Hints
http://hints.macworld.com/article.php?story=2003020406293610