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Palo Alto blocks SMTP Virus with 541 Response

While preparing for some Palo Alto Networks certifications I read something about the antivirus capabilities of blocking viruses via email by sending an SMTP response code of 541 to the sender (link). This was new for me since I thought the Palo Alto would only block IP connections (TCP RST) but not send layer 7 messages (SMTP codes). But actually, it does so by spoofing the IP address of the destination SMTP host. Cool stuff. Of course, I needed to test this. Here we go. 😉

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