To use this Discourse in mailinglist mode, go to your preferences page and there to the ‘Emails’ settings.
You should see something like this:
Enable the mailinglist mode and hit ‘Save Changes’. You will now receive the posts in your email inbox.
Keep in mind that there is a small delay. Discourse waits for a few minutes before sending the email, so that the original poster still has the opportunity to fix errors.
Starting a new topic by email is also possible. Send an email to ‘firstname.lastname@example.org’ to start a new topic in the ‘Help’ category.