HTML injection into emails is a vulnerability where an application accepts user input and embeds it into HTML, allowing malicious users to inject malicious HTML that can be rendered by email clients. This can lead to Cross Site Scripting (XSS) attacks and other security issues. To prevent HTML injection into emails, developers should not embed user input into emails if possible, or always HTML-encode the user input before embedding it into emails. Additionally, they can detect malicious input using regular expressions or other techniques and reject the request. Encoding user input with services like Twilio SendGrid can also help prevent HTML injection attacks. Developers should never solely rely on validating user input and instead always encode the user input before embedding it into code.