A mailgroup can be used to forward mail to multiple different email addresses.
To create a mailgroup, follow this tutorial to create a mailbox, but de-select the mailbox option during creation. This way no mail will be stored on the server that is sent to this address, it will simply be redirected.
Once that is done, click the Mail group icon.
Click Add new member to add addresses that mail will be forwarded to.
Here you can either select email addresses you have already created, or type in an email that is not on your domain. Once you have done that, click OK.