Opt-in vs. Confirmed Opt-in vs. Double Opt-in

If you're new to email marketing, you might not understand the difference between opt-in, opt-out, double opt-in, and confirmed opt-in lists, or all their pros and cons...
Here's a quick run-down of how each one works, plus some pros and cons...
Opt-out: This is an old-fashioned way of building your email list where you'd typically have some form for people to fill out (like to receive a free whitepaper or something). Hidden at the bottom of the page would be a little pre-checked box, with something like, "Yes, please sign me up for your email newsletter!" It's sort of a scummy way of doing it, but technically it's legal. We highly recommend against it, because you'll end up with tons of people who don't understand how they got on your list, who won't read your emails, and who will send complaints to the anti-spam authorities to get your server blacklisted. It's yucky, so stay away from it.
Opt-in: Sometimes referred to as "single opt-in," this method basically means people are never automatically signed up for any email lists. They are only added to an email list if they actually fill out your registration form. While this is much better than opt-out, it's still got its problems. People can sign up friends or family members to lists without their permission. They you end up sending emails to people who've never heard of you. Those people tend to get pretty angry.

Confirmed Opt-in: This is similar to the opt-in method, but after someone signs up for your email list, you'd send them a "thank you" confirmation email that contains a link to unsubscribe from your list (just in case they were signed up by someone else without their permission). On the surface, this method looks a lot better than single opt-in, since it gives people a way out if they never signed up for your list. But think about it. If someone signed you up for an email list that you've never heard of, and you got the confirmation email out of the blue, would you trust the unsubscribe link? Would you have even read the email in the first place?
Double Opt-in: Someone signs up for your email list. You send a confirmation email with a link that they must click before they're added to your list. If they don't click the link, they don't get added to the list. When users confirm that they want on your list, you should store their IP address, and confirmation date and time in your records. This is, in our opinion, the best way to handle your email list. Advantages to this approach include:

• Only people who are truly interested in hearing from you will double opt-in, so your response rates will be much higher (we've seen differences up to 20% higher with double opt-in).
• Since your audience truly wants to hear from you, you can charge more for any advertising that you sell within your email.
• Your competition won't be able to sign up for your email newsletter, then report you for spamming them to the blackhole lists, because you'll have proof they double opted in. Yes, this happens quite a lot.

Disadvantages? Some marketers worry that a lot of people will never follow through with the confirmation link. But we have to ask: if someone's too lazy or unwilling (or incompetent) to click your simple little confirmation link, do you think they're really going to read your emails, or respond to them in any way?