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My guess is that Facebook uses separate domains to ensure their company email accounts don't end up with spam issues due to the overhead of sending out all the notifications for their users.

I've got dozens of "your friends are waiting for you on Facebook" notifications to email addresses that are not even linked on Facebook. I can see where people would mark these as spam.

Also given the less savvy end of the computer user spectrum, I can see where people will just click 'spam' as a quicker alternative to clicking delete and then 'Yes I want to delete'

Finally with the sheer volume of notifications generated, I can see where some overly sensitive mail servers may block the sending domain for too many messages.

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