It's funny but one of my first contact points with iOS (after owning a 3gs some years back) was when I tried to fix the mail settings for a friend and I had the exact same problem described here.
After finding out that you have to add the account via settings it kinda makes sense, but the first thought is: you have an email account, you have an email app, of course you add the account in the app, why would it be anywhere else? The account and the app could not possibly be more directly related to each other.
If you already know how accounts are managed in iOS you will think this is fine, but it is absolutely not intuitive, let alone easy to understand for people who do not know about sso and shared accounts.
After finding out that you have to add the account via settings it kinda makes sense, but the first thought is: you have an email account, you have an email app, of course you add the account in the app, why would it be anywhere else? The account and the app could not possibly be more directly related to each other. If you already know how accounts are managed in iOS you will think this is fine, but it is absolutely not intuitive, let alone easy to understand for people who do not know about sso and shared accounts.