The error indicates that the msn.com address is managed as part of a domain (for example, a work/school or managed Microsoft account) and cannot simply be added as a secondary mailbox under another Outlook.com account. Instead, it must be used as its own separate account in Outlook.
To use the msn.com mailbox in Outlook for Windows, configure it as its own profile or additional account, not as “part of” the existing Outlook.com account:
- Open Outlook for Windows.
- Select File > Add Account.
- Enter the msn.com email address and select Connect.
- When prompted, enter the account password and complete any sign-in or security prompts.
- Select Finish.
This adds the msn.com mailbox as a separate account in Outlook so both mailboxes can be accessed side by side. The msn.com address itself cannot be merged into or treated as an alias of another Microsoft account if it is already its own Microsoft account.
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