A good writeup, but quite shocking that this managed to happen in the first place. I'd have expected that an email service provider would have very good monitoring on deliverability and failure reasons on both sending and receiving, and that something like a cloud migration would be done very incrementally to ensure no loss of service.<p>For this particular issue I would have expected some or all internal email at HEY! to be moved before any customers so that the new system could be tested.<p>Email is notoriously finicky when it comes to networks, IPs, the cryptography involved, and all sorts of details that are in flux during a cloud migration, and it's also notorious for being difficult to recover from if you accidentally get your email listed in denylists.