Okay, that sounds like it's the wrong username or password, you dont have a supported account or the app is broken. Now I'm assuming you have used this app on other MIUI builds?
Well i know i have the Right user name and password. This is my first MIUI Rom so i dont know, But i have used this account on other Rom's..So is there a way to delete the EMAIL. APK and then re-install it?
You have to do the manual setup. I have my yahoo email setup. Google Yahoo email android setup and you should find the right info.
EDIT: Go to manual setup > IMAP account and change to these settings....
Imap server - imap.mail.yahoo.com
SMTP Server- smtp.mobile.mail.yahoo.com