https://elasticemail.com/post/double-opt-in-vs-single-opt-in-email-contacts
Elastic Email | September 05, 2016
You may have heard us mention double-opt in before when we refer to your subscriber lists or contacts. Any subscriber you've added to your list must give you permission to email them, when you use a double opt-in version, this two step process offers you a more complete confirmation that your contact has agreed to receive mail from you.