First of, you need a POP3s enabled server receiving your email. Then make sure to enable "reply_by_email_enabled" and configured the server appropriately in your Admin-Settings under "Email":
Once "email_in" is enabled globally a new configuration option appears in your category settings dialog allowing you to specify an email-address for that category. Emails going to the previously configured inbox to that email-address will be posted in this category.
For spam protection only users of a high trust level can post via email per default. You can change this via the "email_in_min_trust" setting. Additionally, by checking the "accept non-user emails"-checkbox in the category settings, emails to the given email but from unknown email-addresses will be posted in the category by the System-User in a quoted fashion, showing the original email-address and content in the quotes.