[JoshTriplett]

The term "double opt-in" is commonly used by spammers to suggest that this requirement is somehow onerous rather than basic due diligence. Filling in an email address in a form is not an opt-in, as anyone can do that with anyone else's email address. It's necessary to confirm that whoever did so actually owns the email address before you can consider it an opt-in of any kind.

Unsubscribe links are commonly used by disreputable spammers as a way of confirming that the address really exists, so relying on the "List-Unsubscribe" header is not always a good idea.

Yes, some people mark transactional emails as spam. However, far more spammers think their mails were justified when they're not. Your "newsletter" may very well be spam, no matter how much you think it's covered by someone's existing tenuously related relationship to some company you bought a pile of email addresses from.

[carsonreinke]

Double opt-in is an industry standard. Someone buying a list of email addresses is not. Some laws are now enforcing the concept such as CASL.

Either way, newsletters/transactional emails can all be marked as SPAM even though the recipient is legitimate. The sender can be negatively affected by a blind reputation system.

"List-Unsubscribe" for sure can be abused, but better then blindly considering every email flagged as SPAM.

[DanBC]

The industry standard is "confirmed opt in".

Lots of people use the term "double opt it". Some of those people are spammers. If someone cares about sounding credible they should probably use "confirmed opt in" rather than "double opt in".

[carsonreinke]

Clarification, https://blog.mailchimp.com/opt-in-vs-confirmed-opt-in-vs-dou..., http://support2.constantcontact.com/articles/FAQ/1586.

I'm not sure why it is not credible to use the term double opt-in, please explain.

[DanBC]

Mostly it's used by spammers. When you use it there's a hard to shake impression that you don't understand the point about getting confirmation by email from the email address owner, and that you might be using eg checkboxes on a webform as a confirmation.

That mailchimp blog? It's wrong. What they describe as confirmed opt-in is not confirmed opt in, and what they describe as double opt in is in fact just confirmed opt in.

If anyone from mailchimp is reading: please fix this fucking annoying and stupid error.

EDIT: That constantcontact post is correct. Notice how they put "also known as double opt-in" in brackets, and then never use it again but only use confirmed opt-in?