LPIC-2 DNS, Web and Mail Services Practice Question
A mail server running Postfix is deferring messages for a local user. The mail log shows 'status=deferred (mailbox is locked)'. What is the most likely cause?
⚠ Common exam trap
It's easy for candidates to confuse 'mailbox is locked' with permission or quota issues, but the lock message specifically points to a concurrent access conflict, not a filesystem or authorization problem.
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 user's mailbox is currently being accessed by a POP3 client.
The 'mailbox is locked' message in Postfix logs indicates that the mailbox file is currently locked by another process, typically a POP3 or IMAP client that has the mailbox open for exclusive access. Postfix defers delivery because it cannot acquire the necessary lock to write to the mailbox, ensuring data integrity. This is a standard behavior defined by the mailbox locking mechanism (e.g., fcntl, dotlock) used by the MTA and MDA.
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 user's mailbox is currently being accessed by a POP3 client.
Why this is correct
Concurrent access by a mail client can lock the mailbox file, causing Postfix to defer delivery with 'mailbox is locked'.
- ✗
The filesystem containing the mail spool is out of inodes.
Why it's wrong here
Out of inodes would cause a 'No space left on device' error, not a mailbox lock.
- ✗
The disk quota for the user has been exceeded.
Why it's wrong here
Quota exceeded errors are reported as 'mailbox is full' or 'quota exceeded', not 'mailbox is locked'.
- ✗
The Postfix process lacks write permission to the mailbox.
Why it's wrong here
Permission issues typically result in 'permission denied' errors, not 'mailbox is locked'.
Go deeper
Related to this question
About these practice questions
Courseiva writes every LPIC-2 question from scratch — 507 in total, each with an explanation and a wrong-answer breakdown. None are copied from real exams or dumps. Learn why practice questions differ from exam dumps →
JA
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.