The urgent email fallacy is a cognitive bias that results from an overestimation of the efficacy of email. The urgency of an email is determined by the receiver, not the sender.
As an example: Do you email the fire department when your house is on fire?
Probably not.
However, if you were to receive an email from the fire department with the subject "Your House is on Fire" but without the word "URGENT" in it, you would probably still find the timing of that email quite pressing and would spring into action.
The person receiving the email determines how urgent it is. If the person sending the email is important (such as your boss, or your boss' boss), then it will be urgent to you.
Technology has a tendency to be fragile. It doesn't always work, and there is no confirmation that a person received the email (unless it's a corporate network with "Read receipts"). When you email someone with "urgent" in the subject you put yourself in a vulnerable position, whereby every passing moment where no action is taken on the part of the receiver, you become weaker. If something is urgent it's probably best to use a method of communication further down the hierarchy.
It may make sense to write an "Urgent" email at a late hour, but even then the idea of urgency may be based on a 6 or 12-hour timeline. If you need something in five minutes perhaps there is a better method of communication that could be utilized.
In conveying a specific message to a specific person, there is a spectrum. One end is: "this is a very good way to convey information" (ie. this is important), and the other end represents "this is not a good way to convey information" (ie. this is not important). It looks something like this:
Face-to-face interaction -> Video chat -> Phone call -> Text message -> Email -> Social media -> Letter by snail mail -> Homing pigeons -> Message in a bottle in the ocean
Email is essentially one step above Tweeting at someone, and four steps away from going to that person.