To look at the message-queue in EXCHANGE 2013, it's actually rather easy.
- -click START > PROGRAMS > MICROSOFT-EXCHANGE-SERVER-2013 > EXCHANGE-TOOLBOX
- -click QUEUE-VIEWER
Here you will see any messages that are waiting to be delivered. Sometimes a receiving server might delay the message or the receiving server might simply be not available, in which case, the message will wait to be sent again. After a certain period of time, I believe that it's 48 hours, the message will bounce as undeliverable or NDR.