Re: SPAM aware sites: please proof read this eary draft ...

> Stop your users from using false addresses

To hide what they are doing, SPAMers frequently set the sender address
to point at a duff address. For sites using MUAs which submit email
over SMTP, you can avoid your users using an address other than a
valid address within your domain by restricting permitted sender
addresses.

     sender_address_relay = cl.cam.ac.uk

Doesn't sender_verify cover this?