outlook express messages

I have a laptop running XP Pro and Office XP. I have setup a BT Connect dial up internet connection and email account using Express. All works fine.I then connected it to our work LAN,which has shared internet connectivity via a router,which went ok. When I opened Express whilst connected via the LAN the inbox was different and didn’t show any of the emails that had been present whilst using as a stand alone.I then asked an external user to send me an email which arrived ok. I then logged off of the LAN and went stand alone and dial-in using the modem. As before, the mailbox was back to the original, minus the external email that had just been received whilst logged onto the LAN. Any ideas anyone?