問75
頻出度★★☆
PC間で電子メールを送受信する場合に, それぞれの PC とメールサーバとのやり取りで利用される通信プロトコルに関する記述のうち, 適切なものはどれか。
平成21年度 春期 ITパスポート試験 問75
テクノロジ系ネットワーク › 通信プロトコル
This question's explanation and your answers so far.
解説
問 75
Incorrectア
SMTP and POP are swapped. POP is the protocol for retrieving (receiving) mail from a mailbox on a mail server to your own machine, and mail cannot be sent out from a PC with it. Conversely, SMTP is for sending and relaying, and is not used to retrieve mail that has accumulated on the server. The roles are exactly the reverse of the correct answer, so this choice is wrong.
- イ
正解 The correct answer is イ. In e-mail, the protocol used by the sending side and the one used by the retrieving side are different. A sending PC uses SMTP (Simple Mail Transfer Protocol) when it hands the mail it has written over to a mail server, and mail servers use that same SMTP when they relay the mail from one server to the next like a bucket brigade. Mail delivered this way is only stored in the mailbox on the recipient mail server; it does not reach the receiving PC automatically. So the receiving PC connects to the mail server with POP (Post Office Protocol; POP3 is the mainstream version today) and retrieves the mail from its own mailbox. Written as a flow: sending PC —SMTP→ sending mail server —SMTP→ receiving mail server ←POP— receiving PC. Therefore イ, which says that SMTP is used when a PC sends and POP is used when it receives, is appropriate.
Incorrectウ
Saying that POP is used both for sending and for receiving is wrong. As the analogy of picking mail up from a post office suggests, POP is a one-way, retrieval-only protocol that downloads the contents of a mailbox to a PC; it has no function for delivering mail to the other party. With POP alone mail cannot be sent, so mail delivery does not work.
Incorrectエ
Saying that SMTP is used both for sending and for receiving is wrong. SMTP is a transfer protocol that pushes mail on to the next server and delivers it, and sending, as well as relaying between servers, is indeed handled by SMTP. However, the action in which a receiving PC opens its own mailbox on the mail server and takes the mail out cannot be done with SMTP; POP (or IMAP) is required. The description of the receiving side is wrong.
ここがポイント
- The division of roles between the basic e-mail protocols: SMTP for sending and for transfer between servers, POP (POP3) for receiving (retrieving) from the mailbox on a mail server. Memorize them as a pair: SMTP to send, POP to retrieve.
- POP downloads mail to the PC side and manages it there, whereas IMAP (IMAP4) is a receiving protocol that lets you view and manage mail while it stays on the server. When the same mail is handled from several devices, IMAP is more suitable; this difference is also frequently tested.
用語