Which port and protocol is used for SMTP mail transfer (non-submission)?

• A. 25 TCP
• B. 67 UDP
• C. 53 UDP
• D. 80 TCP

Answer: (A)

The key idea is that SMTP mail transfer between mail servers uses TCP because it requires reliable, ordered delivery. For inter-server relaying (non-submission), the standard channel is a TCP connection on the SMTP port used for server-to-server transfer. This makes it the best choice because it ensures messages reach their destination intact and allows proper error handling and queueing.

The other options correspond to different services: DHCP uses UDP, DNS commonly uses UDP for queries, and HTTP uses TCP but for web traffic, not mail transfer between servers. For completeness, client submission typically uses a different path and port (587, often with TLS), and SMTPS on a legacy port (465) is not the default for non-submission SMTP.