Because email has been used in a wide range of ways in the past, including some where genuine email passes through relays which add random crap to the end of th

Because email has been used in a wide range of ways in the past, including some where genuine email passes through relays which add random crap to the end of them. The same applies with email headers: you can't sign all of them, because random headers will be added in-flight. The parameter is a hack to prevent such relays from completely breaking it. But I agree, it should've never been used in the first place because it completely undermines the whole concept of signing email. There should be a very clear separation between signed and unsigned.