Yes, you are right that "maybe" may be a better answer.
However, personal letters and diaries have consistently been found by courts to be copyright protected, and upon the creation of any original work, it is generally deemed that the work is copyrighted and belongs to the author (barring "work-for-hire" or other agreements to the contrary). As copyright holder, then, the author of an email is given the
exclusive rights to reproduce, distribute and display the email, just as a personal letter-writer or diary-writer would have with their writings.
In addition, the posting without permission of any letter, be it email or otherwise, can lead to invasion of privacy actions.
(Just because I am curious, what caselaw do you have on email publication? - since I know you have nothing better to do

)