I am occasionally using a VPN connection and while that connection is up, postfix uses the wrong NIC to try to send email. When there is no VPN connection, postfix uses the primary NIC named enp0s25. At the same time there is another NIC named virbr0 created an used for VirtualBox. In any case when the VPN is connected, the NIC is tun0 but instead of using that or the primary NIC, postfix tries to use virbr0. How do I configure postfix to use tun0, if it is up, otherwise enp0s25? TIA.
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