In my experience no one really uses ST/DT properly in the US in casual email… in fact I can’t remember the last time I received a human-written email referencing EDT or PDT. For some reason everyone just defaults to PST/EST.

I’ve given up even trying and just use “PT” and “ET” or even “NYC time” or “SF time” especially when interacting with those outside the US (who are less familiar with the custom and more confused when a mistake is made).