> But what mystifies me is how these people keep going for years without ever realising they aren't getting account transaction emails (from their banks), notifications of stock trades, even OTP emails etc. How is it possible for people to use these services for years without ever wondering why they aren't getting the routine emails they're supposed to get or even why they aren't getting the OTP email they had generated?

Perhaps they also get paper statements in the mail and text message notifications, and don't even know that they also are being sent redundant emails?

(It can go the other way, too. I think I've got some of my financial institutions configured to only send me email and text. If one was also still sending paper statements and those were going to the wrong address, I'd never notice).