Did you want this server just to do the forwarding, or will people be connecting to it to pick up their mail?
If you were after a fully-fledged mailserver with a lot of features, Zimbra Collaboration Suite is very good. There's an open source edition that's free, and it can do the fowarding you want server-side via the admin interface (https). Users can make use of things like a global address list for the domain if they're using Webmail or the Zimbra Desktop client (free also).
Although you said Linux, I thought I'd mention a Windows mailserver named MDaemon. It has a feature called DomainPOP which I thought was pretty strange but is exactly what you're after. http://www.redline-software.com/eng/support/docs/mdaemon/c13.php
Use DomainPOP Mail Collection (Setup DomainPOP…, or F8) to configure MDaemon to download mail from a remote POP mailbox for redistribution to your users. This feature works by using the POP protocol to download all the mail found in the ISP's POP mailbox associated with the specified logon. Once collected, the messages are parsed according to the settings provided on this dialog and then placed in user mailboxes or the remote mail queue for MDaemon to deliver, just as if the messages had arrived at the server using conventional SMTP transactions.
I worked at a place that was using that and the emails which arrive appear to be from the original sender... so when the final recipient hits Reply, it goes to the intended recipient instead of the mailbox at the forwarding point.