To enable 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 to 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