Email does not require an automatic or instantaneous response. The sender needs to be considerate of the recipient's time. If the message requires immediacy or is of a sensitive nature, email is should not be the communication vehicle anyway. Email is not secure unless it is encrypted and authenticated. Encryption requires both the sender and the recipient to have the encryption program.
Encryption is the coding of the message so only the intended recipient can read.
Authentication is a method to validate the identity of the sender. This is a term meaning to express displeasure to another via email or in a chat group situation. Not following proper net etiquette can cause one to receive hundreds of emails expressing this displeasure. For instance, the NALI list serve encourages all types of communication with the only restrictions being that the message is investigative or NALI related. No jokes or flaming is allowed on the NALI list serve.