The government through the United States Postal Service (USPS) delivers millions upon millions of letters and packages throughout the United States each business day, in which those letters and packages are subject to federal laws and regulations in regards to the right of the government to open up or to record the cover of letters and packages so shipped. For the most part, this means that the average American can mail and receive letters and packages in which the contents of those letters and packages are not known and have not been copied or tampered with by the government.
Then there is email, something that people in this modern world utilized routinely on a daily basis, and quite frankly, something that can have a treasure trove of private information discussed in regards to social security numbers, credit card numbers, names, birthdates, tax information, gossip, pictures, videos, and all sorts of information that most people believe is secure and private and that is specifically meant to be shared with one intended party to another. However, the fundamental difference between email and USPS mail is that the former is pretty much under the control of the terms and conditions of that particular provider's email policies, in which those that utilize that email platform have implicitly agreed to, as opposed to the later being subject to federal laws and regulations.
This does mean that some email providers have not only the right to look at your email but actually make it their policy to do so, not so much because they desire to be "peeping Toms" into your private life, but rather so that they can scrub, analyze, and extract actionable data that is relevant for the creation of targeted ads, of which third party providers are desirous of providing. That said, once an email is opened by any machine or human for that matter, even if such is done for the express purpose of marketing or advertising, it still exposes those that believe that their emails are private and secure to the very opposite of those things, of which, most people would not readily want that to be the case, though, for many email providers, that is actually the case, indeed.
One of the things that so many people forget, is that for the most part it is their responsibility to understand what they are signing up for, when they open up an email account, and if they are unhappy with the terms and conditions of such, to thereby find an alternative email provider, whose terms and conditions, for instance, explicitly protect the privacy and security of those communications. Another thing to consider is that in this hi technology world, when programs and apps are provided to users so often at no charge, that there must therefore be some sort of catch, and that catch usually has a lot to do, with the provider having the right to build a profile on its users, by collecting and aggregating the users' information and thereupon selling it to third parties that have a desire for it.
It should be seen as disconcerting that one's "private" emails are actually being read by outside parties, but that is exactly what is happening in so many instances, today.