ID Vault does not work when a passthru server is in front of the home mail server

When users are disconnected from the LAN they authenticate via passthru servers which then hand off their connection to their home mail servers.We have found that users connecting via a passthru server cannot interact or do anything with the ID Vault.

Has anyone found a way around this without exposing our internal mail servers or creating a VPN?

Subject: Is this passthru server configured to require authentication on that node?