9
An email address that does not match the company
Four seconds of looking, and it catches a lot.
Expand the sender. Look at what comes after the @. A large promotion does not notify winners from a free webmail account, and it does not use a domain with an extra hyphen or a swapped letter.
Display names are free to set and mean nothing. “Prize Department” can sit in front of any address in the world. The domain is the part that had to be registered by someone.
This check is not perfect, domains get spoofed and real companies sometimes use odd subdomains, but it takes four seconds and eliminates a startling proportion of what lands in an inbox.