What type of delivery method uses an email server between the sender and the receiver of messages?

The store-and-forward delivery method involves utilizing an email server to facilitate the transmission of messages between the sender and the recipient. In this method, when an email is sent, it does not go directly from the sender to the receiver. Instead, it first travels to an email server, which temporarily stores the message until it can be forwarded to the recipient's email server. This allows for the handling of situations where the recipient’s email server might not be immediately available, ensuring that messages are queued and successfully delivered once the server is reachable. This process is essential for managing emails, especially in scenarios where network connections may fluctuate or where email servers are experiencing downtime.

The other methods mentioned do not utilize the email server in the same manner. Direct delivery sends messages straight to the recipient with no intermediary server, instant messaging typically involves a real-time communication protocol without storing messages on a server, and client-server refers to a model where client devices communicate with a server, which can encompass various types of data transmission but not necessarily with the store-and-forward method specifically used in email communications.