Thank you, it really had to be a simple check for less technical users to check if the email they received can be trusted.

There is too many email scams that people fall for, especially from banks, airlines, supermarkets etc. So if the email address of the sender comes from an unprotected domain (source) then it could easily fake.

If a domain is not protected with a double DMARC check and set to quarantine or reject above then you can not trust the email you that received.