Email that is not transmitted completely will stay at the transmit center for 24 hours for review instead of going into the user's mailbox directly.
This is to shove most of the junk/spam traffic and some malformed email away on getting into the user's mailbox.
However, user may experience mail lost issue under some network inconsistent conditions, or genuine but badly configured sender servers.
As the network administrator, you get the option to use it or not (it was turned on by default). Login as the admin account (Avomaster/Voadmin account) to turn it off (learn how to do so here).