emails are readable by any of the servers in between the sender and the receiver (which generally neither the sender or the receiver has much control over), and

emails are readable by any of the servers in between the sender and the receiver (which generally neither the sender or the receiver has much control over), and if any of those servers don't support encryption of the link between the servers, so can anyone who can read the network traffic between those servers. Emails also cannot generally be verified, and they can usually be tampered with. Really, any security property you might want is absent from email.