SECURITY
2026-06-04 11:35:43
Thousands of Failed Emails in Track Delivery? Here Is What I Learned
A few days ago, I opened my cPanel Track Delivery report and saw something alarming. Thousands of failed email deliveries. Random email addresses at my domain that did not exist. All rejected with messages like "No such person at this address" and "Number of failed recipients exceeded. Come back in a few hours."
At the same time, I received suspicious single-use code emails from Microsoft for my Hotmail account. And when I checked my domain on MXToolbox, I discovered I was listed on UCEPROTECTL2 – a blacklist. I am not a spammer. I run a legitimate website with custom PHP and MySQL. So what was happening?
After working through this with my hosting provider, I learned I was the target of a Dictionary Attack (also called a Directory Harvest Attack). In this post, I will share exactly what happened, how I stopped it, and how you can protect your own domain.
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