This looks to have probably been a temporary delivery issue that shouldn't be happening any longer.
If you take a look at the error message from your bounce back, the main part to pay attention to is:
[18.104.22.168]:55507 is 550 in an RBL
This means that one of hotmail.com's sending IP addresses was on a black list that our server checks. It got blocked by our spam filters because that IP address had user complaints on SpamCop.net
It doesn't look like their IP address is still blacklisted, and more than likely a new IP address will be used for new delivery attempts anyways. I see successful deliveries between the two mentioned email accounts since 12/29/2012 when that bounce back is from.
I hope that answers your question, if you have any more please let us know!
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