Confirmed Opt-In is used to prevent spam and ensure that only people who specifically request your information are subscribed to your list.
Why use confirmed opt-in?
It prevents people from subscribing false or malicious addresses to your list
It gives you proof that all subscribers did request your information, and protects you from false spam complaints.
As an ISP standard for managing mailing lists, it ensures the best possible deliverability for your messages. Consider the following from Yahoo, possibly the most popular free web based email service:
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