Automatic forwarding and Remote Domains
Remote Domains define a bunch of settings, such as message formats, character sets, and OOFs for messages sent to specified domains outside your Exchange organization. The default Remote Domain setting for the address space * (the asterisk character) applies to all external domains except the ones for which you’ve created a Remote Domain for.
The Allow automatic forward setting for remote domains applies only to client-side forwarding using mechanisms like Inbox Rules. For instance, if a user creates a rule in Microsoft Outlook to automatically forward mail to an external email address, the default setting (for address space *) doesn’t allow it. To enable automatic client-side forwarding of mail to external addresses, select the Allow automatic forward checkbox in a remote domain’s properties.
Alternatively, you can do this from the Exchange Management Shell;
set-remotedomain -identity Default -AutoForwardEnabled $true