Mailing list manager software operated that way almost universally for many years. You sent a mail to xyz-subscribe@example.com and then had to reply to a chall

Mailing list manager software operated that way almost universally for many years. You sent a mail to xyz-subscribe@example.com and then had to reply to a challenge message to confirm the subscription. More recently, it's more about web forms and hosted services and so on, but typically you can't add subscribers on popular mailing list management services without either having the service run that kind of double opt-in check automatically or going through some kind of alternative process that involves explicitly confirming to the service that you have the required consent from somewhere else when adding the addresses directly. There are loads of legitimate mailing lists, and the software and services running them have worked reasonably for decades, you just apparently haven't come across them. Not that I disagree with you that there are plenty of scummy ones as well, sadly.