Access Exchange email of different user

Hi

I’m currently being assigned Admin rights for a small company. My senior asked me if there was a way to access each others email if one of us ain’t here the one present requires info that was sent to the other.
It’s all sweet as we will need to exchange passwords from time to time to gain that access.
I was thinking of using OWA. But we need to do that from inside the network.
I’m no exchange guru, but as far as I’ve heard, we can’t access OWA from inside?!
What other possibilities can we look at?
I’ve got a Windows 2003 Server with Exchange 2003. workstations are XP.
Thanx for your time and help in advance.
W