Describe the steps necessary to configure a custom retention policy in Microsoft 365 to automatically delete emails older than three years from all user mailboxes.

To configure a custom retention policy in Microsoft 365 to automatically delete emails older than three years from all user mailboxes, you need to use the Microsoft Purview compliance portal. First, access the Microsoft Purview compliance portal and navigate to 'Information Governance' > 'Retention policies'. Create a new retention policy. Provide a descriptive name for the policy (e.g., 'Email Retention - 3 Years'). Choose whether you want to retain or delete content, or both retain and then delete. In this case, select 'Delete items automatically'. Specify the retention period. Set the duration to '3 years'. Choose when the retention period starts. Select 'When the items were created'. This ensures that the retention period is calculated from the date the email was sent or received. Choose the locations to which the policy applies. Select 'Exchange mailboxes' and then choose 'All mailboxes'. This ensures the policy applies to all user mailboxes in your organization. Review your settings and create the policy. Once created, the retention policy will automatically start deleting emails that are older than three years from all user mailboxes. The deletion process is performed by the Managed Folder Assistant, which runs periodically on Exchange Online. It is important to test the policy on a small group of mailboxes before applying it to all users to ensure it is working as expected and does not inadvertently delete important emails.