By now it is not possible to set the priority.
We have added this as feature request to our issue database (see #25464: Make priority configurable in the e-mail element).
Because this feature has not a high priority at the moment i give you this workaround:
You can send an E-Mail also per Java.
Easiest way I know is the apache commons mail library. See [here [See here how to use it][1].apache commons mail][1].
Because the priority is not a real internet mail standard you have to set the priority as header like this:
Map headers = new HashMap();
headers.put("X-Priority", "1"); // possible values: "1" (highest) or "2" (high) "3" (normal) "4" (low) and "5" (lowest)
// may you also set the importance header field
headers.put("Importance", "high"); //possible values: "high", "normal" or "low"
email.setHeaders(headers);
[1]: https://commons.apache.org/proper/commons-email/userguide.html