How can I add email forwarding using the Zimbra SOAP API?

You can add email forwarding by sending a SOAP request to the Zimbra server that includes the `ModifyPrefsRequest` or a similar modification, as demonstrated in the Zimbra SOAP API Development Guide 5 - Email Forwarding. The request should contain the `zimbraPrefMailForwardingAddress` attribute set to the desired forwarding email address. To forward to multiple addresses, separate them with commas. Clearing forwarding is done by setting the value to an empty string.