Changes

Once you open the port in the firewall - try the telnet command. You should get a different error this time. This time the problem is that your service isn't listening on the outside interface, it's currently configured to listen only on the loopback (lo) interface.

The first change to the Postfix configuration will be to make it listen for incoming connections on the extrnal intercace, that's eth0 from the VMs point of view.

Restart postfix, confirm (with netstat) that it's now listening on all interfaces, not just loopback, and test connecting to it from the host.

So far you're probably not using vm1 as the DNS server for your host. We'll need to change that for email to work correctly. Normally you don't need an entire DNS server for running an email server, you just add an MX record in your registrar's web interface, but we haven't paid for our domains so we don't have a registrar with a web interface.