There are essentially five levels of permission you need to consider when designing your email programs. Each has their pros and cons and its usage depends on the type of program you are developing and the subscriber experience you want to provide. When trying to decide which one to implement, it might be helpful to assess the amount of risk that is associated with your opt-in program and take into account the relationship you want to build with your subscriber.
You obtain an email address for a subscriber (it doesn't matter how) and begin emailing that person. In order to get off of the list, the subscriber must email or click to opt-out of future mailings. While you may end up with a lot of subscribers on your list, opt-out mailing is the lowest form of permission and is too close to spam to be advisable.
2. Negative Opt-in
You offer subscribers an email subscription form - usually as part of an order form - with a check box that has already been selected for an agreement to receive emails. This person must uncheck the box in order not to receive emails from your company. Again, while you may rapidly build your email file, you could also end up with a lot of angry customers who did not realize that they had agreed to receive your emails. As a result, your mailings may be received as spam.
A subscriber must proactively select a box in order to receive your email communications. Opt-in is the most common form of subscription because it is voluntary on the part of the subscriber and keeps things simple.
4. Confirmed Opt-in
A subscriber opts in for your emails and then receives an email message confirming their subscription and offers them the option to immediately unsubscribe if the subscription was a mistake in any way. This level of permission increases the value of your list, and on some level, protects you against charges of spamming. Moreover, you can use this first email to establish a connection with your new subscriber and showcase the value that they will continue to receive from your mailings.
A subscriber opts in to your email list and then receives an email message from you that requires them to reply to your email in order to be added to the list. This is the gold standard for permission because consumers must essentially subscribe to your list twice indicating that they really want to receive your emails. However, some subscribers may not realize that they need respond to the confirmation email causing you to lose them after the initial opt-in.
Source: Return Path