I used to get the odd one of these when I looked after another site. It was explained to me as spoofing rather than using your mailgateway.
Random genrate the first bit and strap it to @yourdomain and you have a spoof email address to stick into any bulk sender. The recipient who reports will often report it to your ISP rather than the actual header ISP keeping the heat off the spammer for a while.
If there is a link in the spammer's email then they couldn't care less that no-one can email them back because the email doesn't exist. They still get paid their .02 to send that particular email.
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