Each circle is also an email list. The email address is correlated to the name of the circle. For example, the name: 'My Favorite Circle' is the email address: firstname.lastname@example.org. Email users can start a new discussion thread by clicking the "Create New Post" button in the digest email or by sending a new email to that circle email address.
Circle Admins can look-up the associated email address in the Admin Panel. If your circle name has changed there is a chance that the associated email address may be different than the current name in the app.