VPN Portals are located on FortiGate and not on FortiClient. If you do not have direct access you must make this request to the FortiGate manager. I don't think the setting on FortiGate is manageable so if you want to connect you should have it deactivated.
Host check is a sort of filter from the VPN server, the FortiGate. There is a policy that does not allow you for some reason. The client either has to comply to the policy or the policy has to be adapted on the server.
This error means your device cannot connect to your provider's mail server. It can be anything from no network, intermittent or unstable network, or even a good internet connection in general but inability to reach your specific provider's mail server.
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