N10-009 Question 56
Single answerSMTP: 25A company hosts its own email server in a small data center. Internal users can send messages to each other, but messages to external domains remain queued on the mail server. A network technician confirms DNS resolution for external mail exchangers is working and users can browse the internet normally. The perimeter firewall was recently tightened. Which firewall change is most likely needed to restore outbound email delivery to external mail servers?
- A
Allow outbound TCP port 25 from the mail server to the internet
- B
Allow inbound TCP port 25 from the internet to all internal client PCs
- C
Allow outbound UDP port 25 from the mail server to the internet
- D
Allow outbound TCP port 110 from the mail server to the internet
Show answer and explanation
Correct answer: A
Explanation
This scenario tests practical knowledge of SMTP port usage in a troubleshooting context. SMTP uses TCP port 25 for mail transfer between servers, so outbound email to external domains typically requires the mail server to initiate TCP connections on port 25 to remote mail exchangers. If internal mail works and DNS resolution is confirmed, a recently tightened firewall blocking outbound TCP 25 is a likely cause of queued external mail. By contrast, POP3 uses TCP 110 for retrieval, and UDP is not used for standard SMTP transport. This aligns with common operational practice and standard port assignments documented by IANA and reflected in email administration best practices.
- A. Correct.
Correct. SMTP uses TCP port 25 for server-to-server mail transfer. If the organization's mail server can send internally but cannot deliver to external domains, a common cause is that the firewall is blocking outbound TCP 25 from the mail server to remote SMTP servers on the internet. Restoring that specific outbound access would allow the server to establish SMTP sessions with external mail exchangers.
- B. Incorrect.
Incorrect. Inbound TCP 25 would be relevant for receiving SMTP connections from external mail servers, not for sending queued outbound mail to other domains. Also, exposing port 25 to all client PCs would be poor security practice because end-user systems typically do not need to accept inbound SMTP connections.
- C. Incorrect.
Incorrect. SMTP does not use UDP port 25 for mail transfer. Standard SMTP communication is session-based and runs over TCP, so opening UDP 25 would not restore normal outbound delivery.
- D. Incorrect.
Incorrect. TCP port 110 is used by POP3, which is for clients retrieving email from a mail server, not for server-to-server outbound mail delivery. Opening POP3 would not help the mail server send messages to external domains.