LPIC-2 DNS, Web and Mail Services Practice Question
A small company runs a LAMP stack web server with Postfix mail server on a single Ubuntu 22.04 instance. The web server hosts a PHP application that sends password reset emails via the local mail server using PHP's mail() function. Recently, users report that password reset emails are not arriving. The administrator checks the mail log and finds that messages are being accepted by Postfix but are not being delivered. The mail queue shows messages with the status 'deferred'. There are no obvious errors in the mail log. The server has sufficient disk space and memory. The administrator suspects a DNS resolution issue. Which of the following is the most likely cause of the deferred mail?
⚠ Common exam trap
It's easy for candidates to assume a firewall or network issue is the cause when mail is deferred, but the absence of error logs and the 'deferred' status point specifically to a DNS resolution problem, not a connectivity or authentication issue.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The DNS resolver on the server is unable to resolve the MX records for the recipient domains.
Postfix accepts the messages but defers delivery when it cannot resolve the recipient domain's MX record. The mail log shows no errors because the deferral is a normal queue action, not a failure. DNS resolution failure for MX records is a common cause of deferred mail, as Postfix cannot determine where to deliver the message.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The server's firewall is blocking outbound connections on port 25, causing connection timeouts.
Why it's wrong here
Blocking port 25 would cause immediate connection failures, not deferral after acceptance.
- ✓
The DNS resolver on the server is unable to resolve the MX records for the recipient domains.
Why this is correct
Postfix defers mail when it cannot resolve the MX or A record for the destination.
- ✗
The SPF record for the sender domain is missing, causing the recipient's mail server to reject the email.
Why it's wrong here
SPF failure would cause rejection, not deferral, and would appear in logs.
- ✗
The server's IP address has no PTR record, causing the recipient's mail server to reject the connection.
Why it's wrong here
PTR records are not checked during initial delivery; deferral would not occur.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This LPIC-2 practice question is part of Courseiva's free LPI certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the LPIC-2 exam.