Having not been in this position myself, my instinct is to drop the greeting line in emails altogether. I’m sure you considered that and had a good reason not to drop it.
I personally find it somewhat rude, unless it's a follow up message or someone I know very well.
To give a bit of context I help people as a courtesy when the information on my website does not suffice. If readers can't be arsed to say hello before they ask for a few minutes of my time, I quietly wonder who raised them. Imagine walking up to a retail employee and blurting out "I need printer paper" without saying "hello" or "excuse me".
