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.