Outlook.com now uses the BingPreview crawler to crawl links in emails.
This means that one-time login links send to outlook email addresses are marked as used/expired before the user gets the chance to use them, effectively locking them out of the site/their account.
BingPreview currently uses this user agent:"Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/534+ (KHTML, like Gecko) BingPreview/1.0b"