What agent is responsible for directing messages to a specific user in a mail server?

The mail delivery agent (MDA) is responsible for directing messages to specific users within a mail server. Its primary function is to take the incoming mail from the mail transport agent (MTA) and place it in the correct user's mailbox. This involves interfacing with local mailboxes and ensuring that messages are delivered to their intended recipients based on their addresses.

The mail user agent (MUA) typically serves as the client-side application that users interact with to send and receive email. The mail transport agent (MTA) handles the transfer of mail between servers but does not specifically manage the delivery to individual user mailboxes. The application interface agent is not a standard term in the context of mail systems, making it less relevant in this scenario. Therefore, the mail delivery agent is specifically designed to oversee the final step of message delivery, making it the correct answer in this context.