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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.
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On a double opt-in list, all email address 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
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she is the owner of the email address,
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the address is working and
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she indeed wants to subscribe.
Most often, the confirmation action is as simple as replying to the confirmation request or clicking on a link.
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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.