Opt-in is a concept that you will often encounter when you are working with e-mail marketing. Opt-in is used as a term for a confirmed subscription to a newsletter or an email campaign. Thus a subscription that the subscriber performed by themselve.
In Europe it is only allowed to send newsletter to receivers that have subscribed on their own (if you are outside Europe, then please check the specific rules in your country). Hence all the receivers on you list should, as a minimum, have performed an opt-in, i.e. subscribed to the newsletter by themselves.
In many incidents a single opt-in does not suffice. Instead a double opt-in is used. A double opt-in is when a receiver has not only subscribed, but confirmed the subscription as well. This is normally done by sending an email to the receiver which contains a link one has to click at in order to confirm the subscription. This is a double opt-in and it has a lot of advantages.
Imagine that a customer in a store becomes upset about the store. The customer finds the store's email address and subscribes to all the newsletters that the customer is able to find. This will cause the store to receive a lot of newsletter without having signed up for them. If the senders of the newsletters use double opt-in the store will still receive a lot of emails but does simply not confirm their subscription by clicking at the link.
For senders this means that their lists will not be filled with receivers that have not subscribed by themselves. Additionally it reduces the risk of spam complaints as the store never receives a newsletter but just an opt-in email. Had the store, instead, received an ordinary newsletter then the risk of that newsletter being reported as Junk mail is fairly large.
Furthermore opt-in-mails ensures that receivers, who have typed in a wrong email address, are not subscribed to the newsletter. This ensures that you will never send newsletters to a mistyped address which is an advantage as it gives you a more accurate image of the size of your list.
Opt-in in Ubivox
In Ubivox we strictly use double opt-in and it cannot be turned of in ordinary use
If you use API for you subscribtions you have the possibilty of turning of the double opt-in. This is only to be done if you replace our opt-in function with your own. For example this can be done at the subscription to a newsletter through a forum where the receiver already has a user whose email has already been confirmed and where the user is protected by a password so others are not able to subscribe the user to the newsletter.