ITPEC FE Morning April 2021 Question 44

Which of the following is an appropriate explanation of OP25B for email communication?

a)Blocking communication to port 25 to reduce mass-scale delivery of spam or junk emailsb)Blocking emails that are sent to more than 25 recipients at once due to organizational policyc)Blocking Simple Mail Transfer Protocol (SMTP) communication except those sent to port 25d)Blocklisting email addresses that send spams by monitoring email communication to port 25

Explanation

Blocking outbound traffic to port 25 to curb mass spam — OP25B stands for Outbound Port 25 Blocking.

Spam is often sent by infected home PCs that connect directly to other domains' mail servers on SMTP port 25. With OP25B, an ISP blocks connections from its customers' dynamic IP addresses to port 25 on servers outside its own network. Legitimate users still send mail through the ISP's own mail server, or through the submission port 587 with SMTP authentication, so only direct delivery from end-user machines is cut off.

Answer (a)

Why not others:

•(b) — the 25 is a port number, not a limit on recipients; a cap on recipients per message is a separate mail-server setting
•(c) — the reverse of OP25B: it is traffic to port 25 that is blocked, not everything else
•(d) — keeping a list of spamming addresses is a blocklist such as a DNSBL; OP25B blocks by port and does not track senders

Key rule: OP25B stops unauthenticated direct delivery from end users on port 25; legitimate clients submit mail on port 587 with SMTP-AUTH instead.

Security · AI-written, may contain errors

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