The assumption many people make, to their detriment, is to assume that a message sent to their email comes from someone who knows them, or from a reliable source.
After all, one thinks, ‘How would they get my email address?’
Anyone can get your email address. There are sites which find email addresses; do a search, you can find them.
The difference is, in some cases, is that email addresses sold by Facebook or a Site will have a bit more information about you than an average search and make you believe that the writer is legitimate.
Many scammers do a search of your name, find out what schools you attended, where you work, what sites you belong to, etc.
Be very careful.