this is a great explanation of your scenario.
in this blos, Thomas explains the mail api...
it seems like you may be able to change the content type on line 09 under the SMTP section and see if that helps you. I feel like you should be able to but I have not done it myself. I have however, been able to send simple text in email from the XS engine. make sure you have at least SP09 on your HANA system which I think you do.. based on your xsjob process running.
here is also the api documentation in case you need it
http://help.sap.com/hana/SAP_HANA_XS_JavaScript_API_Reference_en/$.net.Mail.html
an alternative would be in SP11... using XS advanced (nodejs) and hopefully there is a node module that would allow you to accomplish this task
good luck!