> It just feels like the baseline expected behavior of a communications system to me that makes no explicit claims otherwise.
Why do you feel like email "makes no explicit claims" about the authenticity of emails? Laypeople are not even aware of the possibility of spoofing the sender field in emails. Technical people can check the "explicit claims" of a protocol like e-mail, SPF, DKIM, etc. to understand what it claims to do. In other words, email makes both implicit claims, and explicit claims about the verifiability of the sender field.
Why do you feel like email "makes no explicit claims" about the authenticity of emails? Laypeople are not even aware of the possibility of spoofing the sender field in emails. Technical people can check the "explicit claims" of a protocol like e-mail, SPF, DKIM, etc. to understand what it claims to do. In other words, email makes both implicit claims, and explicit claims about the verifiability of the sender field.