This circle identifies the sender and is called an avatar.
The avatar will be displayed as a picture or the initials of a contact, taken from the username or email address. [global-string] also provides pre-defined avatars for certain common services, to easily identify these services.
You can personalize the avatar per email address, by using the Contacts application on your device, if you tap on the avatar of the contact | Add/Open | Then proceed to give the contact a profile picture, all incoming emails thereafter will have this shared avatar.
You can change the size of the avatars by tapping More (o o o) | Global Settings | Customize Appearance | Design Preferences | Contact Picture Design
To remove this image/avatar from Mail-List view, please follow these steps: More (o o o) | Global Settings | Customize Appearance | Show Avatar | Toggle OFF