If you are not sure about the exact definitions of the various levels of opt-in when it comes to building email marketing lists or other types of email lists, here is a quick review of what each of them mean. Because there is some confusion about what each of them means, especially where one method of list-building may have more than one name, we've created this resource explaining the generally accepted definitions of Double Opt-In, Confirmed Opt-In (COI), Single Opt-In, Opt-In, Opt-Out, and Transactional email for you. These are the definitions that the inbox providers, ISPs, and spam filters use, and so it's important for you, as an email sender, to know them.
