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Instead of deleting the user completely which will also remove all data under the user's account, you can decommission the user so that the mailbox is still accessible. So long as the user's mailbox is 25GB or less, this can be done without the need for a license. Once this process is complete, you can forward or share the mailbox to another active user. Note: This will only preserve the Mailbox data and not any data in the user's OneDrive.
Navigate and login to [[https://admin.microsoft.com]]
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Click on "Active Users", then Search for the user you wish Decomission. Then click on the user.
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Click on the "Mail" tab, then click on "Convert to Shared mailbox"
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Click "Convert" to complete.
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Click on the user again.
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Click on the "Mail" tab, then click on "Manage global address list visibility"
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Remove the check next to "Show in my organization address list" and click "Save changes" After which you will click the "X" a the top right corner to close the panel.
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Click on the user again.
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Click on the "Licenses and apps" tab. The remove check marks next to all licenses and click "Save change" After which you will click the "X" a the top right corner to close the panel. Now the user is decommissioned.
*Note: Shared Mailboxs have a limit of 25GB at the free tier, if the mailbox is larger, it will require a Exchange Online Plan 1 license.*
