How to apply a retention policy to multiple mailboxes in EAC (Exchange admin center)

Step 1: Sign into Office admin portal via http://portal.office.com and open Exchange admin center

Step 2: Navigate to Recipients > Mailboxes.

Step 3: Use Ctrl key to select multiple mailboxes.

Step 4: In the details pane, click More options.

Step 5: Under Retention Policy, click Update.

Step 6: In Bulk Assign Retention Policy, select the retention policy you want to apply to the mailboxes, and then click Save.