N10-009 Question 46
Single answer1.4 Explain common networking ports, protocols, services, and traffic types.A systems administrator is deploying a secure email solution for remote users. The company requires users to send email from mobile devices without using an unencrypted legacy submission method, and the firewall team needs to allow only the appropriate outbound port from the mobile device network to the mail gateway. Which port should the administrator request be opened?
- A
TCP 25
- B
TCP 110
- C
TCP 143
- D
TCP 587
Show answer and explanation
Correct answer: D
Explanation
The correct answer is TCP 587. In a real-world deployment, client devices typically use SMTP submission on port 587 to send outbound mail through a mail gateway, usually with authentication and often with STARTTLS to encrypt the session. TCP 25 is intended mainly for SMTP relay between mail servers, not for modern client submission. TCP 110 and TCP 143 are retrieval protocols for POP3 and IMAP, respectively. This aligns with common email deployment best practices and IETF guidance, including RFC 6409 for Message Submission and RFC 8314, which discusses secure use of email submission and access protocols.
- A. Incorrect.
TCP 25 is primarily used for SMTP server-to-server mail transfer and is historically associated with message relay. While some clients may try to use it, modern best practice is not to use TCP 25 for authenticated client message submission from end-user devices. Many providers also restrict outbound TCP 25 to reduce spam.
- B. Incorrect.
TCP 110 is used for POP3, which is a mail retrieval protocol, not a message submission protocol. Opening this port would not allow users to send email. A candidate might choose this if they confuse inbound mail retrieval with outbound mail sending.
- C. Incorrect.
TCP 143 is used for IMAP, which is also a mail retrieval and mailbox access protocol. It allows clients to access and manage messages stored on a mail server, but it is not the standard port for secure client mail submission. This is a common distractor because mobile email clients often use both IMAP and SMTP together.
- D. Correct.
TCP 587 is the standard port for authenticated SMTP message submission by email clients. It is the appropriate choice when users need to send email securely from mobile devices without relying on legacy unencrypted submission practices. STARTTLS is commonly used on this port to protect the session.