Any news regarding this issue?
For me it's obviously a Mail.app bug. I keep receiving email with unreadable fonts in the subject line when sent from Microsoft outlook.
Today I made another test: I sent a message from hotmail and from gmail to another account that I use in Mail.app.
The fonts in the subject were capital greek in both messages.
The message sent from the gmail account was ok.
The message sent from hotmail had some arabic style fonts in the subject line. Please note however, that the same message appears with the correct fonts not only through webmail, but also in my iOS mail app!! It's clearly a mac os mail app bug and strangely enough Apple has not figured that out yet many years now..