A lot of spam uses fake return addresses. So back around 2000 it occurred to someone that if there were a way to validate the return addresses in mail, they could reject the stuff with bad return addresses.
A straightforward way to do that is a callout, doing a partial mail transaction to see if the putative sender's mail server accepts mail to that address. This approach was popular for a few years, but due to its combination of ineffectiveness and abusiveness, it's now used only by small mail systems whose managers don't know any better.
What's wrong with it?
I was dismayed to hear just last week from an acquaintance who was trying to persuade his system manager not to use this brand new anti-spam silver bullet. I hope he succeeded.
Related topics: Spam
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