ACLU blog: The Government Cannot Force E-mail Companies to Copy and Save Your Account ‘Just in Case’ by Melodi Dincer & Kristin M. Mulvey:
Paper letters have a final resting stop — whomever they are addressed to. From a practical standpoint, and a legal one, that feature of regular mail made understanding and applying privacy protections relatively straightforward. But as our communication technologies have changed, courts have struggled to grant a similar degree of privacy protection to communications in the modern era.
Digital communications present new problems. For example, your email does not live in your letterbox but in an online repository operated by a private company. And as digital communications like email and social media become more ubiquitous in society, investigators increasingly rely on them as important sources of evidence.