Retention policy specifies the number of days and months you would like to retain mailboxes for each user account. The retention policy allows you to configure your mailbox to delete emails at a time specified by you. Therefore, we recommend that you use retention policy setup to manage retention of your mailbox and space usage. Perform the following steps to setup retention policy for shared mailbox.

1. Login to Office365 using your administrator credentials.
2. From the left hand navigation pane, click Admin.
   
   Microsoft 365 admin center page appears.
3. On the Microsoft admin center page, click Show all to expand more options.
   
   After clicking Show All additional options related to admin centers appears.
4. Under Admin centers, click Exchange.
   
   A new page named Exchange admin center appears.
5. Under compliance management, select retention tags.
   
6. Click on the + icon and select applied automatically to entire mailbox (default).
   
7. On the new page, enter the following information to assign the tag to the retention policy.
   •   Name
     In the Name field, assign the name to your newly created retention policy.
   • Retention action
     Under Retention action, choose the Permanently Delete option.
   • Retention period
     Under Retention period, choose the option When the item
     reaches the following age (in days) and enter the number of days. 
     Set the number of days you would like the mails to remain in your shared mailbox.
   • Comment
     Under the Comment section, you can add any comments regarding the retention policy.
8. Click Save.
   
9. Go back to Exchange admin center, and click recipients.
   
10. Under mailboxes tab, select your newly created shared mailbox. The General Setting page for the shared mailbox appears.
11.   From the left menu bar of your shared mailbox, select mailbox features.
12.  Under the role assignment policy section, choose Delete older emails of shared mailbox.
13.  Click Save.

After this setup is complete, emails will be automatically removed from the central inbox at a predetermined frequency which is defined while creating the Retention tag.