Each facebook user gets legitimate facebook emails with a fixed
header. I.e., Bob will always get mails from ABC123, and Wilma from EFG456.
There is no way a phisher / spammer would know what that was, unless one inadvertently published it. Thus it is a great header to filter real and fake mails with. (Odd that twitter notifications don't use such a fixed number too.)
OK, let's say one has inadvertently published it. Now what https://www.facebook.com/settings?tab=security setting can one click to update it to a new one? (I assume that there is none, as it would probably be seen as too confusing to the masses.)