Is there no way to encrypt or sign e-mail inboxes on the server in such a way that it is tamper-proof? Most solutions focus on preventing individual e-mails to

Is there no way to encrypt or sign e-mail inboxes on the server in such a way that it is tamper-proof? Most solutions focus on preventing individual e-mails to be read or modified by a 3rd party. Does anyone apply similar techniques to a whole mailbox / account?