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Looking in the bounces.php script, I see you define a hard bounce as three soft bounces. But does Amazon send a different SNS for a soft bounce vs. a hard bounce? After reading this:
It seems they manage the soft bounces themselves, and will notify with a hard bounce when it finally gives up.
So does your bounce script really need to process the soft bounces?
Amazon's documentation provide responses for both hard and soft bounces. So I made sure I handle both in Sendy. I only read about this blog post later. But I thought I'd just leave soft bounce handling alone, there can never be too many checks when it comes to bounce handling.