i'm pretty sure Mox (email server with included webui written in Go) does that - at least the Umlauts in mails i get from Hetzner seem to always stand out.

it also defaults to not loading HTML in emails, which i love. really opened my eyes to how dumb it really is to just accept all kinds of dynamic content in unknown messages. (kinda same as how the modern web relies on remote code execution to work)