How do I send an HTML email?

Set content type. Look at this method.

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

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