plain text support – MIME_HTML_ONLY

I am checking my email deliverability and it is flagged because there is no plain text version of outgoing emails. It’s my understanding that WordPress does not support this, however since this plugin is a gateway to the SMTP server, it seems that you could easily implement an option to create a text only version. I know there are plenty of PHP libraries that already do this like phpmailer. If you could add that feature it would be awesome.