I had a number of users informed me that their messages were "stuck in the queue or something hanging." Their concerns were confirmed after using the mailq command. After a brief investigation, it was discovered was a small hiccup with DNS.
Here's an example of the mailq output.
# mailq
or
# mailq -v
/var/spool/mqueue (19 requests)
----Q-ID---- --Size-- -Priority- ---Q-Time--- ---------Sender/Recipient--------
l9HDvk500367 5 120046 Oct 17 22:57 user1
user2
l9GGhCW02706 1333 7770119 Oct 17 01:43 user3
user1
l9GGhCX02706 1363 7770119 Oct 17 01:43 user4
user5
...