Mail Privacy Protection settings and VPN

Hello:

I was reading about Mail Privacy Protection because a customer issue case, and how it don't work with VPNs. The final user sees all the time a message on the email top, and then the final user claims to the vpn owner.

Which are the hostnames to set in the VPN as whitelisted to remove the message from there?

Thanks in advance

Mail Privacy Protection settings and VPN
 
 
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