Yes! When one of your collaborators in a project posts a discussion, uploads a file or adds a comment, you will immediately get an email notification (assuming you have your project notification preferences set to "Real Time Updates"). Once you receive that email notification, simply hit reply in your email client (whether it's Gmail, Yahoo, Hotmail, or anything else) - just like replying to any email.
Then, type a comment that you would like to post to the project in reply to the notification you received and hit send. That's it! The comment will be posted to the project and will appear as a reply to the original discussion.