Yes, the safe assumption is that Apple reads all your mail. Not that any other provider is better---most are probably worse.
For a company who hangs their hat on privacy, it is a dang shame that their native mail client can't support PGP/GnuPG. I mean it'll do S/MIME, but who uses that regularly.
I don't think they read all of it. Probably just metadata. But would be good to know what's being sent.