> But they do not discriminate against who issued it. Valid? All good. Just like it with email. Ahaha of course they do! Try paying online with a credit card i

> But they do not discriminate against who issued it. Valid? All good. Just like it with email. Ahaha of course they do! Try paying online with a credit card issued by a bank from Uganda, or by a bank that allows customers to prepay their credit card bills, or with a "privacy.com" type virtual card, and you will very quickly see that payment processors most certainly do not treat all cards the same, even if they are valid. And yes, that's "just like with email". Because the majority of online services will outright reject addresses from anonymous/"temp mail" providers, even though they are perfectly valid and can receive mail. Many will additionally disallow +suffix addresses, because they can be used to create multiple accounts from the same mailbox, and also make it easier to automatically get rid of spam originating from the service provider. Service providers want the user's primary, personal email address. Exclusion rules exist to increase the likelihood that this address is in fact what the user has entered. The standard validation flow is clearly not sufficient, else service providers wouldn't bother with all the additional complexity.