Most people have a morning routine. Checking your email happens to be a staple of most people's mornings. At times the morning is halted by a rogue email from someone we don't know. Most people will just delete these emails instead of taking a risk by opening them. Many viruses are transmitted through email attachments. So its no surprise that this practice has become accepted.
The problem with the deleting of all those emails comes with the risk of deleting something important. When you delete all unknown or unidentifiable you could be passing on opportunities or missing out on important information.
The solution may come in the form of a reverse email search. With a reverse email search you may be able to identify the owner of the unknown email address, safely and without taking unnecessary risks.