Steps to set up email forwarding in Office 365 to an external email address without creating a mailbox

Step 1: Login to Office365 admin panel

Step 2: On the left sidebar under Admin click on Exchange

Step 3: Click on Recipients in the left sidebar

Step 4: Select Contacts and then click on the arrow next to the + and Choose Mail User

Step 5: Fill in the New User Details

Step 6: Click Save option