Official comment
Single opt-in allows you to start mailing contacts immediately, while double opt-in requires that people verify their email address by clicking a link in email that’s sent to them. We recommend learning the pros and cons of each before choosing. Learn more.
The advantage of double opt in is that you know the customer wants to be included in your mailing list, and you will get fewer opt outs, bounces and complaints. But there can be some problems with double opt in:
- Do not use it if the customer has purchased something like a membership or other product; you won't be able to supply them with the membership details until they opt in! This can lead to customer service complaints because they have actually paid for the membership.
- Be careful using it for limited time promotions; some people may not opt in for days and miss the deadline.
You can also set the opt in to "Double Opt In Optional" which allows you to email them between the time they first opt in and their acknowledgement of being double opted in. This would allow the membership details or special limited time offer to be sent and still help you build a confirmed, double opt in list.