In general, unless you and your correspondents are using PGP or some such, your email is readable by anyone who can obtain a search warrant in the country where your email provider resides. (Protonmail may or may not be an exception ).
However there are some other worrying changes like the fact that TANs and TARs are secret and have no judicial review. Warrants (even the new computer access warrants that were passed in the same bill) have judicial review. But at the end of the day, they'd be serving a warrant to fastmail, not you.
Personally I use mailbox.org, and one of the really nice features is that you can give them a PGP public key and they'll encrypt everything you receive. So in the case of a warrant (though Germany has different laws on that matter) they could, at most, get the contents of new emails.