The user bob complains that he cannot access his email. In which directory would you look to see if there is any deliverable email for him?

A.
/var/spool/mail

B.
/var/mail/mqueue

C.
/var/spool/mqueue

D.
/home/bob/.mail

Explanation/Reference:
A) in /var/spool/mail/ (or /var/mail in newer distributions) exists one file per user, this file contains all (undelivered) mails for that user. If the user is unable to read his/her mails, check the permissions on the file.
B) ???
C) The mail queue, /var/spool/mqueue, is the directory in which the mail queue and temporary files reside. The messages are stored in various queue files that exist under the /var/spool/mqueue directory. Queue files take the following forms:
qf*—control (queue) files for messages
df*—data files
tf*—temporary files
nf*—a file used when a unique ID is created
xf*—transcript file of the current session
Normally, a sendmail subdaemon processes the messages in this queue periodically, attempting to deliver each message. Each time sendmail processes the queue, it reads and sorts the queue, then attempts to run all jobs in order.
D) ???