Works for both incoming and outgoing here, you have to define the host in astaro in networks then you can add it.
Of course you have to have the ASL box setup as the primary MX record for the domain and its only going to work for emails that the box is responsible for handling. The usual things really.
Perhaps if you are a bit more specific about what you want to do I can advise further, but I use it for incoming and outgoing routes for certain mail servers internally.
