K-9 Mail v6.802 not sending traffic via connected VPN

I had a network meltdown a month ago, just before the latest release, and it’s taken me a while to get everything back the way I had it, so it is only now I am able to notice that my K-9 Mail app has upgraded to 6.802 and is now not sending traffic via my connected VPN.

I am running my own VPN server to funnel all my mobile traffic via my home network.

This means I am able to monitor the traffic on my VPN from the server end.

I also have a mail server running inside my home network, which is not accessible from the Internet.

When I connect to the VPN while connected to my home wifi, K-9 Mail is able to connect to my mail server, but I see no relevant traffic on the VPN interface on my router.

When I disconnect from the Wifi and am using Mobile data, K-9 Mail is not able to connect to my mail server at all (either incoming or outgoing), and I see no traffic on the VPN. I can see traffic on the VPN for other connections to internal servers, but nothing from K-9 Mail.

I didn’t realise that Android apps needed to obey OpenVPN connections, but it seems it’s a per-app thing.

I can get K-9 to connect via the tunnel by editing my VPN settings to force all traffic to go via the gateway, but I’m pretty sure that I hadn’t needed to do that previously.

It depends on how the app’s network instance. The ones using the IS instance just use whatever the OS uses. The ones with their own implementation can, but are not forced to, use that. Thus, some apps can ignore your VPN connection, e.g. TAN apps from many banks ignore VPN.