As the admin of an Office 365 organization, you might have company requirements to set up email forwarding for a user’s mailbox.
Email forwarding lets you forward email messages sent to a user’s mailbox to another user’s mailbox inside or outside of your organization.
Configure email forwarding
You must have admin permissions in Office 365 to do this.
Go to the Office 365 admin center.
Select recipients in features pane and select mailboxes tab
Double-click on the User mailbox to open up the properties. Click the mailbox features tab. Scroll down a bit. Under Mail Flow, delivery options click view details.
New page for delivery options will open. Here, check the option, enable forwarding.
Click browse and select the user account and click OK
Check the option, deliver message to both forwarding address and mailbox. Then click OK button.
This option will retain messages to the mailbox and forward copies of the messages.