This is how you unread an email on Loop;
Just click on the empty dot on the left of the email and the dot will become blue, which means email is unread.
If you opened an email and read it and you want to mark it as unread, simply click on the 'unread' icon on the top as shown below:
You can mark an email as unread directly from the inbox. Just select the email you would like to mark as unread and swipe from left to right.
If that email is already opened, then tap on the 'mark as unread' icon on the top of the email and then choose 'Mark unread'.