Opt-in Mailing Lists and Safelists
Email addresses on a single opt-in list are not confirmed. Anybody can submit anybody's address to the list, and it will be there until it is unsubscribed. This will be treated as spam.
On a double opt-in list, all email addresses must be confirmed before they are added. A request for confirmation is sent to the submitted address and the address owner must take some action to confirm that:
(s)he is the owner of the email address
the address is working
(s)he indeed wants to subscribe
Most often, the confirmation action is as simple as replying to the confirmation request or clicking on a link.
A safelist is a group of people who have agreed to receive emails from one another. Usually, the group requires membership and has rules regarding how often you are allowed to email the members.