Many people have heard of these terms but are not sure what this means. Below is your answer,
- 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.
- 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
- she is the owner of the email address,
- the address is working and
- she indeed wants to subscribe.
- (Ref: http://email.about.com/cs/marketingtips/qt/et060202.htm)