I can confirm some of the issues Gizmo is reporting
For the most part, the message avatars work great. I don’t know what source is being used, but they are much, much better than those that Apple Mail uses (which are more contacts book-based).
That being said, I have never experienced the wrong avatar being used for a sender, but I have seen the avatars go (for lack of a better description) from a premium-sourced to looking stock-sourced. For example, an image avatar would be replaced by a letter from time to time.
I “believe” that, based on prior conversations I have had here on this issue, it’s a sync issue or an update process that occurs from time to time.
But again, on the whole, the avatar source that is being used is more universal and robust than what I have seen other email clients like Apple use.