Have you checked your postfix configuration?

Edit:
Spent too long re-reading the posts before typing my response

I am learning Linux networking.
Can you recommend a good way of learning Linux networking?

When I set inet_interfaces=all, and did this:

both did not work. Meaning I could not telnet from host.
I had to reboot the VM.
Strange. Why?


To telnet into smtp form host to VM (guest) to work,
I also had to disable firewall,
or include postfix daemon in appropriate zone.

virbr0 did not work since it is in DOWN state.
How do you turn it to UP state?

However virtual ethernet (eno16777736) works:
I learned a whole bunch for this post.
Thank you everyone for your generous time.

Thank you for your help.

One more thing I learned.

To get telnet prompt do this:

Using the escape key switches from session mode to command mode. You can change client parameters and your not talking to the server anymore until you switch back.