I think you drew the opposite inference from what I intended. Postal mail is more protected because the Post Office is (was) an organ of the government. As an organ of the government, it is restricted by the 4th amendment as to what it can do with your mail.
E-mail is different because the intermediaries are non-governmental third parties. The idea is that you do not give up your expectation of privacy by giving mail to the Post Office to deliver, because you know the Post Office is limited by the 4th amendment, while you do give up your expectation of privacy by giving mail to your e-mail provider, because you know your e-mail provider has no obligation to protect your privacy.
E-mail is different because the intermediaries are non-governmental third parties. The idea is that you do not give up your expectation of privacy by giving mail to the Post Office to deliver, because you know the Post Office is limited by the 4th amendment, while you do give up your expectation of privacy by giving mail to your e-mail provider, because you know your e-mail provider has no obligation to protect your privacy.