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