It is much easier to thumb through emails using Fastmail since most emails have an avatar. Proton only uses favicons which either don't show up or are low quality, even with popular domains like Google. Instead, Proton should use BIMI logos or Gravatar when they are available, then fall back to favicons when it's the only option. Since some people might have privacy concerns with Gravatar, that should be optional or it can be routed through Proton's servers similar to how they do with favicons.