Windows 10 Outlook will not connect over VPN

Been seeing this issue, hope someone else has solved. When my windows 10 users are connected to VPN, outlook will not connect. No errors, just tries to connect and goes to disconnected. Network drives work properly, hostnames resolve, just Outlook not connecting.

When not connected to VPN, Outlook connects fine. However then users do not have network shares etc.

Have not seen this on any Win7 or 8 users.

Details - Windows 10, Office 2013, Watchguard VPN, Exchange 2010.

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