I don't know how sensible this is, but it has worked as a last resort with a client.
Set up a Workgroup postoffice on a Win9x machine, and copy the WGPO subdirectory whole to the Win2k machine. You will have to do the admin from a Win9x machine, but the mail clients can communicate directly with the Win2k machine. You will also have to make sure that Microsoft mail is set up in Outlook, as it won't work with the internet mail services.