How Does the [[domain]] Placeholder Work?
The [[domain]] placeholder, when used within phishing templates, landing pages, and training notifications, will automatically populate the domain of whoever is receiving the phishing email. This is true even if you have users across multiple domains on your account (such as yourdomain.co, yourdomain.net, etc.).
Note that you should NOT modify the [[domain]] placeholder. For example, do not enter your domain within the brackets, like [[yourdomain.com]]. Doing so will actually prevent your phishing emails from being delivered. If this occurs, you'll see a "bounce" in your phishing reports, indicating that an error occurred and that the "email address does not comply with RFC 2822, 3.6.2."
See more about placeholders here: How to Use Placeholders