Surely spam filtering/rejection should be done by the MTA, preferably during the SMTP protocol conversation so as to prevent the black-holing of legitimate e-mails (i.e. the sender doesn't know it's not been delivered) and the prevention of joe-job collateral spamming?
It should also be noted that, at least in the UK, the act of an MTA at the final delivery mail server accepting an e-mail means that it's legally been received. Hence, any legal document which the MTA accepts has been deemed to have been legally served even if no human ever reads it.
Please be careful, e-mail administration can be a legal minefield!
Steve
Computer Systems Administrator, E-Mail:-steve@earth.ox.ac.uk Department of Earth Sciences, Tel:- +44 (0)1865 282110 University of Oxford, Parks Road, Oxford, UK. Fax:- +44 (0)1865 272072