Email that is explicitly requested by the recipient.
The definition of opt-in email has been a matter of intense debate.
“single opt-in” vs. “double opt-in”
The term single opt-in simply means that actions were taken to sign up for the email in question. The term double opt-in means that the subscriber has actively confirmed their subscription, typically by responding to an automatically-generated message sent to the email address. Proponents of double opt-in may not actually use that term, as they feel any email labled “opt-in” must be verified.
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Opt-This, Opt-That: Putting Permission to Rest
Small Business Computing (October 18, 2002)
Six Ways to Build an Opt-In List (From Zero)
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Opt-In: More Than Permission to Send
Small Business Computing (September 13, 2002)
Whatever Happened to Opt-In?
ClickZ (January 25, 2001)
How Much Opt-In Is Enough?
ClickZ (September 4, 2000)
Speaking The Opt-In Language
ClickZ (November 22, 1999)