Reply to a community email from your email program and your reply will post 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'll get an email about it. (To understand how email notifications work, see Customizing Email Notifications).
To reply by email, just click Reply in your email program and then type your discussion reply just as you'd type a reply to any email; then send the email. If the community is set up to receive replies via email, your reply will be posted as if you'd logged in to your community and posted it from there.