What is the mailing list?
Your mailing list shows everyone who has subscribed to, or unsubscribed from your mass messages. This includes members, and those who don't have accounts. Your mailing list can be found in Mass message > Manage mailing list.
Although recipients can unsubscribe themselves via the unsubscribe link in the footer of emails, you can also do this for them. This can be done via the menu:
Can I import a list of recipients?
Not at this time. Contact support if you would like to import a mailing list.