Double opt-in

A form takes an address, and that is a claim rather than a fact. Double opt-in turns it into a fact by sending one mail to the address itself and asking for an action from inside it: until that action happens the signup is pending and nothing else is sent. Single opt-in skips the step and starts sending immediately, which is faster in every respect except the one that matters, since it cannot tell the difference between an address someone typed for themselves and an address someone typed by mistake.

The mechanism is the same one magic link uses, put to a different purpose. Both prove possession of a mailbox by requiring an act performed from inside it, and the difference is what the proof buys: a magic link buys a session, a confirmation buys consent. That is worth keeping straight, because the two get built by the same team out of the same sending code and then described as if they were one feature. One is authentication. The other is a record that somebody agreed.

The cost is real and it should be stated plainly: a confirmation step loses subscribers. Some never open the mail, some open it and do not act, some find it in a spam folder. Every one of those is a name a single opt-in list would have had, which is exactly the argument for the step rather than against it. The people lost are overwhelmingly the ones who would never have read anything, and what remains is a list whose open rate describes its readers rather than its size.

The case that settles it is the typo. An address off by one letter usually belongs to somebody, and under single opt-in that somebody starts receiving mail they never asked for, cannot recognise, and can only stop by using an unsubscribe link in a mail from a company they have never heard of. The same goes for an address typed into a form by a third party, maliciously or as a joke. Confirmation is the only step in the whole flow that can tell those apart from a real subscriber, which is why it belongs to ethics rather than to deliverability, even though deliverability is the argument that usually wins the meeting.