Hotmail and Outlook.com have long supported linked email accounts for organizing messages. However, Microsoft now sees connected accounts as tempting targets for hackers -- so tempting, in fact, that the company is severing those links as a safety measure. Within the next two months, Outlook.com will move to using its alias system as the only way to handle multiple accounts. Users will have options to forward email and send messages from other addresses, but they won't get to control multiple accounts through one sign-in. Microsoft will start unlinking accounts in late July, so those who'd like a more orderly transition to the safer (if less convenient) approach will want to act quickly.
Source: Outlook Blog