I have had the opportunity to use a Windows Mobile 5.0 device with the Messaging and Security Feature Pack for Windows Mobile 5.0. This includes a new feature called Direct Push (you can read more about that here: http://blogs.technet.com/exchange/archive/2005/06/07/406035.aspx).
The side effect of this is that I was getting e-mail faster on my phone than when I was sitting at my desk. I hadn’t noticed this delay before, but now it was annoying. I was getting a chirp from my phone and then a few minutes later, the e-mail would finally show up on my laptop. After looking into this, I found that this is caused by the security in Windows XP SP2’s Windows Firewall. Lucky for me, I found this KB article that details the issue better than I could describe in this blog, and also includes instructions for adding an exception for Outlook to the firewall so that the server can tell it there is new mail. You can read that article here: http://support.microsoft.com/default.aspx?scid=kb;en-us;839226
Does it really matter that I now get e-mail faster on my laptop? Not really, but at least it doesn’t bug me anymore.