In same vein some e-commerce sites and payment processors secretly distrust commercial email services and don't tell you that they've blacklisted your email provider.

You go to make a car or flight reservation, enter all kinds of info including your credit card, and then bam, it says "Oops, something went wrong, try again later". Or an e-commerce site asks you to enter your email and they'll send you a confirmation code, but the confirmation email never comes.

I've determined by experiment--using a different IP but a Gmail email address--that a frequent cause is a secret bias against commercial email services. It's particularly infuriating because I think a commercial email user with a paid account is a much lower fraud risk than a free Gmail user.