Calling them either "Idiots" or "Dumb users" seems reasonable, so why not roll with it?
Would you call someone who can't drive a car an idiot? Or someone who is using an ATM machine for the first time? These people are not competent. They are inexperienced. But a pejorative term like "idiot" isn't called for.
If you can't apply the term "idiots" to these people then the word is useless and can never be used.
I would if this person was out on the highway. I learned to drive on a parking lot, and then on roads with very little traffic. People who can't drive are a danger to themselves. People who are too incompetent to know what their correct email address is are a danger to themselves too.
Sure, if they didn't even attempt to learn to drive before getting behind the wheel.
Or someone who is using an ATM machine for the first time?
Sure, if their failure to use even the most elementary mental faculties available to them affected anyone other than themselves.