If you're subscribed for email notifications to content, you can post a reply simply by replying to notification email. For example, imagine you're subscribed to notifications on a discussion thread. When someone posts a reply to the discussion, you'll get an email about it. Reply to the email, 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 and posted it.
When posting this way, be sure to leave the special code at the end of the subject line. The code will be numbers and letters in square brackets, like so: [Xxxxx-xxx-xxxx].