When you offer free content on your website, you are providing value. In order to recive this value, you request an email address to add to your list.
A single opt-in is a one-step process—an email is entered into an opt-in form. No confirmation is required, and they immediately become a subscriber.
A double opt-in involves a two-step process where an email is entered into the opt-in form, then a confirmation email is sent. The email is not added to your list until the subscription is confirmed.
The key difference is that a single opt-in doesn’t require a confirmation, and a double opt-in does require a confirmation.

An opt-in is a form on a website where you ask for an email address to add to your mailing list. Usually you also offer a free gift or service in exchange for valued access to your client’s inbox.
MailChimp conducted a study and found that 61 percent of people never completed the double opt-in signup process.
“In email marketing, senders are required to get permission from their contacts before they can send promotional emails to them. Many countries around the world have laws that require permission, or consent, before email marketing communications can be sent. CAN-SPAM in the United States and CASL in Canada are two examples of such laws, and there are many other international anti-spam laws worldwide.
Any time someone fills out a form that says something like “By submitting this form, you agree to receive email marketing from My Company,” we call that “opt-in.” The person has opted in to receive content from your company.
Rules vary by country, but they all have one main point in common: you need to get permission from anyone you send marketing email to.”
The Importance of Permission
Terms of Use and Anti-Spam Requirements
About Your Contacts