A single opt-in agreement occurs when a person provides their email address to a company or service only once. You sign up to receive a newsletter from the company, but you never receive a confirmation email from the company or website. You are considered a single-opt in person even if provide your email address at a company or store's physical location. Single opt-in lists are more likely to report spam because they usually only have a passive interest in a company's product or service. Also, they may report emails as spam, because they accidentally signed up to receive a company's newsletter through a third party.
A double opt-in agreement occurs when a user signs up for a newsletter or information about a company and then the company sends them an email that confirms their subscription. Email marketing companies like to make sure that users Double Opt-in, so if the company is reported for spamming they have a record of that user Double Opting-in therefore, demonstrating their interest in the company's product or service. Users whodouble opt-in to a list are more likely going to actively participate in email offers than those who single opt-in making them valuable members of a mailer's subscriber list.