Double opt-in
From ExtractValue Wiki
Double Opt-in is when a user has subscribed for a newsletter or other email marketing messages by explicitly requesting it and confirming the email address to be her own. This is usually done by responding to a confirmation email sent to the email address in question.
This eliminates the chance of abuse where somebody submits somebody else's email address without their knowledge and against their will. Or where a random opt-in request is activated by bots.
