A college was asking me the other day about enabling external email forwarding for Exchange Labs.  This is not a feature that is on by default, but you can enable it in a few minutes by logging on to your service through PowerShell, creating a runspace, popping onto it and then entering:

set-remotedomain * -autoforwardenabled $true

That's it.

Mailbox users can then use rules to set up mail forwards to other services by simply typing the address to send mail to into the rule.

Enjoy!

Jonny