The problem with filtering before storing in the end users mailbox is as
Ned
says, the concept of the mail envelope. At the SMTP level, the envelope
probably has many recipients, so who's configuration do you use? It's
only
when you are about to put a message in a users INBOX will you know that
you
can take action on behalf of all the recipients (cos there is only one).
Hence when MailSite rejects a message at the SMTP level it is using a
global
server level script.
You can reject per Recipient and accept others. Nothing wrong with it.
You can do this per domain or recipient, the recipient could modify this
configuration option and the data could be in a SQL or LDAP server.