Re: REJECT-ON-SMTP-DATA (Re: Mail Server best practices - was: Pandora's Box of new TLDs)

one note about whether to filter at receiving SMTP server or later. The receiving SMTP server is the one that has the conversation with the sender. Rejecting mail from servers having an un-backtranslatable IP is best done right away by the receiving server right after the HELO command by issuing error message about the IP being unbacktranslatable. Reduces the load. later on (for instance at the client level), you need to parse the RFC822 text header and there are some bits that are missing, notably the RCPT TO: commands. This is especially true when the "TO" in the 822 header is faked. Blocking messages as early as possible also greatly reduces the load on your system, disk storage requirements etc.