You need some plug-in for sending HTML mail.
On Windows, you can send HTML mail without plug-in using CDO in vbscript.
You can do this by sending the content as html. The native send email function in FM does text only, but as user19752 suggests, you can do this with a plugin as well as other methods. There are several good plugins that are capable of sending multipart mime type emails, allowing you to send html and text alternative in the same message.
You can also send via a web based script, either your own php to relay through your email server, or a service with an API like Mandrill or Mailgun.