Why does Outlook.com put validated emails in the junk folder?

I look after a website with a membership section. When people are registered with the site an activation link is sent to them, however hotmail and now outlook.com either dumps them completely or puts them in the junk folder where they get deleted after ten days.

I just registered myself as a test member using my hotmail (now outlook.com) email address. The received message header shows that it  passed all the validation checks and yet smartscreen has decided to put it in the junk folder, saying "This message is in your junk folder because you don't seem to want emails from this sender." but I have never done anything to stop messages from the sending website. I know the message is genuine because I caused it to be sent by registering a member. What on earth is the point of going to the trouble of validating a message if you are going to ignore the result anyway?

Here is the header validation section (redacted) from the received message:

Authentication-Results: hotmail.com; spf=pass (sender IP is xx.xx.xx.xx) smtp.mailfrom=*** Email address is removed for privacy ***; dkim=none header.d=xxxxx.org.uk; x-hmca=pass header.id=*** Email address is removed for privacy ***
X-SID-PRA: *** Email address is removed for privacy ***
X-AUTH-Result: PASS
X-SID-Result: PASS
X-Message-Status: n:n

This is absolutely infuriating. I'm told that lots of members never get their activation emails. This situation clearly cannot continue!