The account avatar is used to show which folder the message is in, when looking at unified folders like the Favorites Unread folder. It shows on the right below the time.
It doesn’t have anything to do with the email address for the sent message. Instead, eM Client sources contact avatars from the Internet. If you use Gravatar to add an avatar for the email address you are sending from, then that avatar will show on the left next to your email address in sent messages. Recipients who have this function in their email applications will also be able to see that avatar when they receive your messages.