How email templates work:

// 1. Build a Thymeleaf Context with variables
Context ctx = new Context();
ctx.setVariable("name", "Ana Kovač");

// 2. Process the HTML template into a String
String html = templateEngine.process("email/welcome", ctx);

// 3. Create a MIME message and set HTML body
MimeMessage msg = mailSender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(msg, true, "UTF-8");
helper.setTo("student@example.com");
helper.setSubject("Welcome!");
helper.setText(html, true);   // true = isHtml

// 4. Send
mailSender.send(msg);