The opt-in confirmation email is sent after a subscriber has opted-in to one of your lists via a sign-up form. The reason for sending this email is to confirm that the subscriber wants to receive email from you as part of that subscription.
Sending unsolicited emails (or spam) to subscribers that have not opted-in to your email list is a violation of email marketing practices in many countries and can result in your sending domain becoming blocked by anti-spam servers around the globe.
For more information about designing and creating your opt-in confirmation email, see the support topic, How to Setup and Customize an Opt-in Confirmation Email.
It is possible to turn off the double-opt-in via API.
The code example illustrates how this can be carried out via API call to the platform (as indicated by the highlighted text in the code) using a programming language such as PHP. In order to turn off the opt-in confirmation, the “double_optin” variable must be set to “FALSE”
It's important to note, though, that turning off the opt-in confirmation email requires an API key. For more information about using APIs, see the Support Topic How Do I Generate and Use API Keys.
To learn more about API please see the API Documentation Page.