When you convert a user's mailbox to a shared mailbox, all of the existing email and calendar is retained but can be accessible to any users with delegated access to the mailbox.
Note: The user mailbox you're converting needs a license assigned to it before you convert it to a shared mailbox. Shared mailboxes can have up to 50GB of data without a license assigned to them.
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Open up a web browser to admin.microsoft.com and login with your credentials.
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On the left side select the exchange admin center.
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Under recipients click mailboxes.
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Select the user that you would like to convert and on the right side click convert.
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Click shared to confirm that the mailbox is in the list of shared mailboxes.
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If the mailbox is smaller than 50 GB, you can remove the license and request that we lower your license count for billing.
Note: We recommend that you block sign in for the shared mailbox that you create to keep the account secure.