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Why have IP Addresses for suspicious mail? Shouldn't you just refuse to send it?

Facebook applies filtering to all messages submitted by users. If we are confident that a message is abusive, we refuse to deliver it and take appropriate action on the user's account. However, some messages fall into a gray area and we deliver them but mark them as spam. If any of these gray area messages are destined for other mail systems, we route them out of a set of IP Addresses dedicated to such traffic as a signal to other administrators. We also add 'X-Spam-Flag: YES' to the message header and DKIM sign them as spc.facebook.com. Facebook encourages the administrators of other systems to treat these messages with an appropriate level of suspicion by delivering them but marking them as spam or by taking whatever other actions they feel are appropriate.