Hotmail not receiving e-mails

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http://serverfault.com – I have a server that can't send e-mails to some Hotmail accounts. How could I fix this? For Gmail accounts, it sends well. I saw the headers of an e-mail sent to a Gmail account and can confirm that SPF and DKIM are correctly set. I also checked here if the IP is blacklisted and it wasn't. What else could it be? (HowTos)