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Module 5: Application Layer & Security

20242m

When the mail server sends mail to other mail servers it becomes ____.
(i) SMTP client
(ii) SMTP server
(iii) Peer
(iv) Master

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Answer: (i) SMTP client

Delivery of mail between mail servers uses SMTP as a push protocol. The server that has mail to send opens a TCP connection to the receiving mail server (port 25) and issues the SMTP commands - HELO/EHLO, MAIL FROM, RCPT TO, DATA - which makes it the SMTP client of that connection, while the receiving server is the SMTP server.

A mail server therefore changes role with the direction of traffic: it acts as an SMTP server when a user's mail client (or another server) connects to it to submit or deliver mail, and as an SMTP client when it forwards/relays that mail onward to the next mail server. It also runs a POP3/IMAP server so users can retrieve their mail. Peer and Master are not SMTP roles.

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