If you’ve configured your forum settings to send messages to you in email, you can reply to email and the reply will automatically get posted back to the forum – in other words, it will work just like the QNA email list in that regard. However, you cannot start a new topic from email – you have to go to the forum to start a new topic (i.e. ask a new question).
I did notice that if you reply to an email from the forum, you should not mess with the reply address in any way. That apparently confuses discourse and in that case your reply doesn’t get posted to the forum. I noticed that @jim replied to a private message I had sent him and added himself as a cc: on the reply – his reply did not get posted back to me (by chance I found it in an email log). So if you reply to a forum email, don’t modify the reply address at all, even to add a cc or forward.