According to SpamHaus, confirmed opt-in means that the recipients has confirmed permission for the address to be included on the specific mailing list, by confirming (responding to) the list subscription request verification.
Confirmed opt-in ensures that the recipient subscribed using a valid email address and with the email address owner’s permission. It legally protects the email sender because the subscription confirmation received from the recipient proves that the recipient intentionally subscribed and grants permission for the mailings.
With that in mind, we designed our WPNewsman plugin so that it allows to collect subscribers around a WordPress site using the confirmed opt-in or double opt-in method. However, some users believe that the single opt-in method is better because it does not require an extra step from the subscriber while you still have the proof of the subscription fact.
Below we’d like to shatter the most widespread myths about the confirmed opt-in method and explain why we implemented confirmed opt-in in WPNewsman.