Reply to a community email from your email program and your reply is posted directly into your community.
If your community manager has turned on this feature, you can post a reply to something by replying to a notification email. You first need to set your user Preferences to receive email notifications on a particular stream, so that when someone updates something you follow in that stream, you get an email about it. For more information on how email notifications work, see Customizing notifications.
If the community is set up to receive replies via email, your reply is posted as if you'd logged in to your community and posted it from there.