Using the avatar area of the OP to clearly define if there is something strange with the email.
the user can choose different layouts of the avatars, for office, default, mailing lists, ads, girlfriend(s) ... if you have a conversation the avatar layout shrink, but you have always the same avatar layout in each conversation. you can choose the avatar layout for a mail adress, a mail server (company, e.g. @kde.org, ...)
instead of the color gradient you can also choose the company logo (like you have it for your music) e.g say all @kde.org mail adresses get logo kde or all @deutschebahn.com get the db logo, ...
I like the idea with the faded colorized avatar in c. In the breeze icon set the additional information is in the bottom right and left corners like mail-signed-full, mail-signature-unknown, I think this action icons were used to sign mails, ... so maybe the avatar could have the same language style (emblem icons in the corners)