api, #cybersecurity, #webdev, #python On July 15 I queued a campaign to fifty addresses I had already "verified" with a plain SMTP handshake. Every single one returned 250 OK . Three hours later, twelve of them bounced. That's a 24% failure rate in 38 minutes on addresses that looked perfectly healthy in the logs. The code that reproduces the failure is on GitHub . This time I wanted to know what the SMTP handshake had hidden. So I ran the same list through a validator that reads MX beh...