Microsoft has long supported a linked accounts feature for its Hotmail and Outlook.com service, allowing users to simply switch between Microsoft Accounts without having to enter passwords. The feature was originally introduced in 2006, but Microsoft says it plans to stop supporting it over the next couple of months.
Microsoft blames security for the change of heart, noting that users would often keep their primary account password up-to-date, but failed to regularly change the password on associated accounts. The software maker found that malicious users have been compromising secondary accounts to get access to a full primary account.
Read more at: Microsoft killing off Outlook.com linked accounts feature in favor of aliases | The Verge
See also: How to Turn "Two-step Verification" On or Off for your Microsoft Account