When your comment on a pin receives a response from a user, you will receive an email notification. You can reply to that message from the email in one of two ways:
Launch the link in the email
Then you can click on the “Reply” button and click on the “Post” icon to submit the reply message.
Alternatively, you could directly reply to the email. Your message will be posted in Yellowdig as a response to the specific comment that you received the email notification for.