If you are using the SpamCop realtime block list as part of your anti-spam measures, be warned that they appear to be blocking several legitimate Facebook mail servers.
I’ve seen them blocking the following Facebook mail server IPs:
Presumably there’s a whole block of IPs that SpamCop is blocking. Since I like to get e-mail from Facebook, I have currently disabled my SpamCop RBL check. You may want to do the same.