Recieved HTML messages display as html text not formatted

I’ve just started using mailspring. I’ve received message in HTML.
When I try to read I see them as plain text. i.e. the mark up tags etc not as a formatted message.
The rest seems to ok, so far.
This is pretty basic so I assume its a pref option some where.
Also when I send emails to others sometimes I want to send some HTML.
How do I do that.