This circle identifies the sender, called an avatar.
This will be displayed as a picture or the initials of the contact, taken from the username or email address. BlueMail also provide pre-defined avatars for some common service, to easier identify these services.
You can personalize the avatar per email address, by using your Phone Contacts application, 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.
To remove this image/avatar from Mail-List view, please follow these steps: More | Global Settings | Customize Appearance | Show Avatar | Toggle OFF