spam blocking using iptables

[Mark Olliver]

Hi all

can anyone think of a rule which check's if mail is coming from a valid
domain? & or block mail from certain users/ domains.
also can we create iptables rules to block mail with certain messages, or
can't we see this at the packet level

Thanks

Mark

http://www.carsplus.co.uk/e-Card/mpo.shtml