There are many reasons why you should not open an email from an unknown source.
1.) The email may send a virus or harmful content to your computer, ruining it.
2.) There may be threats or unsafe material in the email
As you can see, opening an unknown email can be extremely dangerous. Try to stay away from them and do not open links from unknown emails.
I hope this helps you. Please let me know if you need any more help. Have a great day!