Network Protocols and Packet Switching: Question 1
Syllabus 14.1
A city library's email system uses several different application-layer protocols to handle its members' messages.
A librarian finishes writing a message in the library's email app and sends it. The message is passed on from the library's own outgoing mail server to the mail server that looks after the recipient's mailbox, ready for the recipient to collect it later.
Which protocol is responsible for this step: passing an outgoing email message on from one mail server to another?
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Worked solution
Why C is correct
The scenario describes a message that has just been sent, moving from the library’s own mail server on to the mail server that manages the recipient’s mailbox. This is a mail server relaying an outgoing message onward, not a user downloading or reading stored mail.
SMTP (Simple Mail Transfer Protocol) is the protocol used to send email, both when a mail client first submits an outgoing message to its own mail server, and when one mail server relays that message on to the next mail server closer to its destination. This exactly matches the step described, so C is correct.
Why the other options are wrong
- A (POP3) is wrong: POP3 retrieves mail that is waiting on a server, typically downloading it to one device. It does not relay outgoing messages between mail servers.
- B (IMAP) is wrong: IMAP also retrieves and manages stored mail, keeping it synchronised across devices. Like POP3, it is a retrieval protocol, not a relay protocol.
- D (FTP) is wrong: FTP (File Transfer Protocol) transfers files directly between a client and a server. It is a separate application-layer protocol from any of the mail protocols and plays no part in relaying email between mail servers.
Final answer
- C. SMTP is the protocol used to send and relay outgoing email, including passing it from one mail server on to the next.