As i've been hunting a ghost for the past weeks, i just want to add that all those posts from the past talk about directly calling an email client template by passing in the sys_id of the template.
This does NOT work (anymore).
servicenow decides which template to call and you have no influence over that.
If you need a specific template to be called, you need to get creative, for example setting a (hidden) field value temporarely, like so:
function openEmailClient()
{
var state = g_form.getValue('u_supplier_state'); // save the current value
g_form.setValue('u_supplier_state','send to supplier'); // change it to something unique so you can build the email client template condition
emailClientOpenPop('incident_task'); // open the email client and let servicenow find your template
g_form.setValue('u_supplier_state',state); // reset, so servicenow finds the default template for all other calls of the email client on that table
}
