I think you can even port forward with the free plan of ProtonVPN.

You can find many public hosted instances here but some of them use Cloudflare because they fear DDoS attacks. So keep that in mind when choosing your instance. Best is to host your own instance.

Why the need for a specific mobile app for Mailbox? I use Mailbox in combination with FariEmail app on Android. It even supports PGP. Couldn’t be happier.