Unfortunately FileMaker only supports plain text for email. I struggle with this as well as my users would like to send out formatted emails.
Apparently, the best plugin is the 360 plugin, which sounds great if one is sending from the client or one-off FMPro.
But, if one is using from the server, then it has to be time-triggered loops for everyone using the datatase, sending and receiving every X minutes. In a server environment, might as well just do it with PHP and have httppost send data to the PHP templates, which in turn send the email(s).
Serious design flaw for Filemaker, who admits that > 50% of their apps are CRM, to leave out year after year.