How do I send an HTML email?

As per the Javadoc, the MimeMessage#setText() sets a default mime type of text/plain, while you need text/html. Rather use MimeMessage#setContent() instead.

message.setContent(someHtmlMessage, "text/html; charset=utf-8");

For additional details, see:

• GMail Media Queries
• GMail CSS Design
• CSS support in mail clients

Set content type. Look at this method.

message.setContent("<h1>Hello</h1>", "text/html");

If you are using Google app engine/Java, then use the following...

MimeMessage msg = new MimeMessage(session);
msg.setFrom(new InternetAddress(SENDER_EMAIL_ADDRESS, "Admin"));
msg.addRecipient(Message.RecipientType.TO,
                 new InternetAddress(toAddress, "user");

msg.setSubject(subject,"UTF-8");

Multipart mp = new MimeMultipart();
MimeBodyPart htmlPart = new MimeBodyPart();
htmlPart.setContent(message, "text/html");
mp.addBodyPart(htmlPart);
msg.setContent(mp);
Transport.send(msg);