Unless things havew changed not without a plugin...
"When I was young I thought that money was the most important thing in life; now that I am old I know that it is." --- Oscar Wilde
Since FileMakers standard method mail sending scripts does not support html mail, you will have to find another way.
1) Have a service doing this to you. Since sending mail require internetconnection you can use a FileMaker robot to generate and send the mail.
2) Something could probably be done with the webviewer?
I would use this with Filemaker Server. FM Go creates an email record in the database and when done sets a flag. This flag triggers a script on the server which sends email. For example with our MBS Plugin.