To enable the automatic forward of all your email from your Apple iCloud mailbox:
Log into your iCloud mailbox
Click the Action pop-up menu in the sidebar, then choose Preferences.
In the General pane, select Forward my email then type the forwarding address in the text field.
If you don’t want to store a copy of each forwarded message in iCloud, select Delete messages after forwarding.
To forward only specific emails, see Filter email automatically.
Reference: iCloud: Forward all messages automatically