On 12/10/14 15:53, lee wrote:
And when they are filtered, does the sender get a message telling him that their message hasn't been delivered?
The requirement in RFC 2821 (the successor to RFC 821 which you've recently been referring to) section 4.2.5 that a server which issues a 2yz completion status after final dot must return appropriate notification to the sender cannot be fulfilled is an unreasonable demand in the modern era.
If you wait for all filtering to complete before issuing a response to final dot, your mail server is vulnerable to denial-of-service attacks unless you undercook your filtering; if you issue a 2yz status before filtering is complete, then send the notifications demanded by RFC 2821 if the deferred filtering results in a rejection, your mail server is now a public nuisance because of the amount of backscatter it generates.