Note that Microsoft actually DID violate user privacy by reading hosted emails when they thought it would help them catch a leaker. Institutionally they have never had a history of ethical behaviour regardless of how much their founder now gives to charity.
An employer provided email is another matter. Superficially all email ought to be company business in which at least one party is acting as the agent of the company. In reality personal matters are probably frequently intermingled even between two employees of the org providing the email. What's public and what's private becomes complicated.
If there is a dispute do you ask both parties to voluntarily turn over emails? In a dispute between a party inside and outside of the org do you make do on what you can get voluntarily turned over?
Do you merely make do without such evidence? Does one go to court and request such if needed from your own org? If that is the standard how does one enforce proper disclosure when there is no adversarial situation?
So does Yahoo/Verizon/Oath (recent HN thread) for commercial reasons (data-harvesting, targeting, ads). Google did this too until 2016, apparently. Note: automated, so probably not literally reading in person..
Most employee manuals I have read actually directly state this in their company policy as an agreement of use of company email and internet.
That is now relevant in the whole EU for example and it is a very sensible rule.
Maybe I am the conservative here, but I wouldn't expect my provider and mail provider to look into my private communication. Even if it is 100% their infrastructure.
Maybe we shouldn't expect it, but having a legislative insurance certainly helps.