It was a bit trickier in my case because it was an English tool to fill a German form. There was the awkward issue of "Frau Dr." and "Herr Dr." being the same thing in the English dropdown.

The other issue is that the full name is used in the address, but only the last name in the greeting. There are no apartment numbers in Germany, so the name in the address must match the name on the mailbox. There can also be a lot of "Herr Müller" working at the same address.

In the end I opted for Mr/Mrs/Other + first and last name, and dropped the title. It's good enough for this purpose, but it shows how tricky something like a name can get, especially if the person filling the form is not aware of all this.

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".