Opt-in and opt-out
Two opposite ways of building a list: opt-in writes only to those who said yes, opt-out writes to everyone who has not said no.
Opt-in asks for agreement before writing: an unticked checkbox, a form filled in, an email confirmation. The person acts, and that action is the consent.
Opt-out assumes the opposite: you write first, and whoever no longer wants it removes themselves. In Europe this is not an option for prospecting private individuals — consent must come first.
The word survives in two legitimate places nonetheless: the unsubscribe link, which is an opt-out and is required in every message; and prospecting to business addresses, where legitimate interest can apply provided the person is informed and can object.
Why it matters
It is the difference between a list that grows slowly and gets read, and one that grows fast and ends up in spam: complaints almost always come from people who never asked for anything.
In Plumail an address comes in by agreement and leaves in one click. Signup forms ask for email confirmation before letting anyone in, and every campaign carries its unsubscribe link plus the one-click unsubscribe header. An address that leaves enters the suppression list: no re-imported file can put it back.