We have an SBS 2011 box (with Exchange) in a central location. A remote office has a LAN to LAN VPN connection. The SBS box location has an ADSL connection, and therefore we have set up a smart host for SMTP sending.
When email is sent (from Outlook, connected to Exchange) from the remote office, it is invariably rejected as spam.
Email is not marked as spam if a software (Windows PPTP) VPN is established directly to the SBS box (regardless of whether the LAN-to-LAN VPN is connected or not).
I believe the issue is something to do with the originating-ip being detected as the remote office IP (which is dynamic) - but would have expected that this should not behave differently in either scenario?