Reputation can go wrong, legitimate emails with double opt-in still can be marked as SPAM by the user. Gmail definitely is slowing that down by offering a warning to the user when there is a `List-Unsubscribe` header. Even that still does not work and probably the reason by AT&T bill is sometimes marked as SPAM.
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.
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.
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".
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?
Emails with double opt-in still can BE spam. The act of signing up for an email list does not give the email list owner the right to send any amount of any email content to you.
Traditionally I consider SPAM only unsolicited emails, any double opt-in that is too high of volume that the user could be able to opt-out. Users marking as SPAM and decreasing reputation because of that seems like the wrong target.
Sometimes I opt-out of email lists, but then I keep getting email from then, probably because I was on multiple email lists and only opted out of one -- altohugh when I subscribed, I subscribed only once.
Sometimes I try to opt-out, but they ask me to login, and I don't remember my password because they asked me to put weird symbols and uppercase letters on it, while my normal login-everywhere password does not have these.
Sometimes I try to opt-out, but the link is broken.
Sometimes (this is what happens most of times) I am subscribed automatically to email lists whenever I sign up to some website. Shouldn't this be considered spam? I did not receive a confirmation email -- or maybe I did, but the confirmation email was to confirm my account on the site, not my subscription on that email list.
Sometimes the sender forgets to his opt-out link.
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The question that these cases pose is: what is the difference between "spam" and "email that can be useful to others but that you don't want to receive"?
And the answer is: SPAM, as explained in the original submission, is a global uncustomizable tag, if something is spam, it is spam to everybody, not just to you. That is not the ideal situation. We could do better, but I don't think it will be better within the email protocol, since it would be impossible to Google to calculate the spam-probability of each message according to its receiving user. The only way it to move to other protocols.
