Which of the following characteristics cannot be used in smart content?

Smart content in HubSpot is designed to tailor the user experience based on specific characteristics of visitors, ensuring that the content they see is relevant to them. Characteristics such as device type, referral source, and preferred language can all be utilized to dynamically change content based on the user's context, allowing for a more personalized interaction.

For instance, device type can help determine whether to present a mobile-friendly version of a web page, while referral source can indicate where a visitor has come from, which allows for adjustments in messaging that resonate better with that traffic source. Similarly, preferred language can enhance user experience by displaying content that matches the user's language preference.

However, an email address does not serve as a characteristic that can be used to personalize content. While an email address is crucial for identification and segmentation purposes, it doesn't inherently modify the content displayed on a webpage. Instead, it's typically used to identify a user or contact in broader marketing contexts rather than as a specific smart content criterion. Consequently, the other options listed directly inform how content is customized for viewers, whereas an email address functions differently within the realm of content management.