Exchange server in VPN

[Guest]

[iomega55]

I have a wide area network, this WAN is a wide VPN. In a segment A)11.254.1.* exists a exchange mail server. In my LAN(which is part of the WAN) I have 2 segments B) 12.1.12.* and C) 172.1.12.*.   The Endian box is pluged in the segment B)

Which kind of configuration do I need to apply in my Endian 2.4 for having total access(send and receive emails) in my Outlook Client from segments B and C?