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The issue is that MUAs and MUA-level filters like SpamAssassin cannot reliably determine the envelope sender (note; not recipient) at intermediate steps along the Received chain. Some forwarding use cases, like mailing lists, fetchmail, or SRS-compliant forwarders, will resend the message with a new envelope sender, which means that a filter cannot check back beyond that step. Given situations where spam arrives via lists or at a forwarding address, this means that the filter cannot use the env-sender to detect spam in that case, and in the fetchmail situation, the filter cannot trust that data at all, and therefore cannot use env-sender or SPF at all. – JustinMason
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But [SpamAssassin] sometimes runs as an MTA-level filter - \[http://marc.merlins.org/linux/exim/sa.html| example at the top shows SA refusing mail at SMTP time\]. It should run as such wherever possible, IMO, and when it does, it should reliably determine the envelope. -- [MatthewElvey] |