To send e-mails in the HTML format first define the text of the e-mail as HTML document template (see the “Adding/Editing Document Templates section). After that add an email notification as described in How to configure the system to send e-mails. This notification will have the predefined HTMLMessage attribute of the Document type. Set initial value of this attribute to the name of the HTML document template. Then define a rule to send this e-mail as usual.