Which of the following agents is responsible for moving a message from one user to another?

The mail transport agent is specifically designed to handle the transfer of electronic mail messages between users. It is responsible for facilitating the communication by ensuring that messages are routed and delivered from the sender's server to the recipient's server. This involves determining the path the message should take through various networks, making decisions based on factors such as routing tables and protocols.

A mail user agent, on the other hand, is more focused on the interface that allows users to interact with their emails. It is what users engage with to compose and read their messages but does not handle the actual transit of messages.

The mail delivery agent plays a different role in the process, as it is responsible for delivering messages to a user’s inbox once they arrive at the destination mail server. It does not move messages between users but rather ensures that they are placed in the appropriate mailbox for the recipient.

An application interface agent does not specifically address mail but could refer to a broader concept involving software that interfaces with users or other applications. In the context of moving messages, it is not focused on the email passage itself.

Therefore, the mail transport agent is the correct choice as it encapsulates the essential function of transferring messages across networks from sender to receiver.