Getting off Outlook.com's junk mail block list
Microsoft's Outlook.com is one of the largest email providers in the world and anybody who sends email to customers will encounter plenty of outlook.com, hotmail.com or live.com addresses. As such, Microsoft has to deal with a lot of spam for all those billions of emails it processes and sometimes the spam detection is a bit overzealous. It doesn't take much for your server's IP address to be blacklisted, or you might even inherit a blacklisting from the previous owner of the IP address assigned to your newly commissioned server. Then you get the 550 SC-001 error.
<...@live.com>: host example.hotmail.com[xx.xx.xx.xx] said: 550 SC-001 (XXXXXX-XXXXXX) Unfortunately, messages from yy.yy.yy.yy weren't sent. Please contact your Internet service provider since part of their network is on our block list. You can also refer your provider to http://mail.live.com/mail/troubleshooting.aspx#errors. (in reply to MAIL FROM command)
Really annoying if you are trying to send a confirmation email to a customer who just spent hundreds of dollars on your site. Unfortunately, while the link in the error message tells you well enough what's happening, it doesn't provide you with a whole lot of options on what to actually do to get yourself off the blacklist. For that, you need to fill out a support form, which is extremely well hidden. To save you the trouble of searching for it, here's the link: https://support.microsoft.com/en-us/getsupport?oaspworkflow=start_188.8.131.52&wfname=capsub&productkey=edfsmsbl3
Make sure you're not actually sending spam (if you are, go do the world a favour and stick your tongue into an exposed electrical socket), fill out the form, and wait for Microsoft's response. They're usually quite prompt in responding when your claim is valid.
Oh, and while you're at it, sign up for the Junk Mail Reporting Programme which will help you idenitify issues with the email sent on behalf of your domain. Note: you'll need a Microsoft account for this and if you only have a personal account, you might want to sign up for one for business use.