It seems with this update the push/real time option in the mail frequency setting has been removed. In 6.9.8 this setting still exist when i'm using gmail and would be removed when i use other mail address. While in this build, the option for push mail doesn't show up no matter what mail provider i'm using.
I don't want to use the gmail / inbox app, since in the miui it seems that the push mail on both of those app doesn't work correctly. I always get the mail about 10 minutes after the sender sent them, while on the default mail app, it actually push the mail to me in real time.