Maybe you didn't know that Facebook offers email address similar to this one (username@facebook.com) when you sign up. Every email that anyone sends on this email goes in your Facebook's message inbox. But now Facebook is going to change behavior of this service a little bit.
Soon they will forward that facebook email address to your primary email address. This means, you'll get emails sent to your Facebook email in your email inbox e.g. Gmail, outlook or Yahoo mail etc.
As you are going to receive emails on your primary email, so, go and update it to the one where you wanna receive emails. This email forwarding feature is not live yet but when it does there'll be option to opt-out of this feature. That will make sure no one can send you email messages through your Facebook email.
This feature is similar to the one that Google Plus introduced earlier. (Send emails to your gmail connections whether you know their email adress or not)
