This means mail is not being sent to other servers. This can be explained by outages or performance issues with the network or remote servers.

This indicates a performance issue or outage on the local server. Reasons could be slowness in consulting AD, slowness in handing messages off to local delivery or SMTP delivery. It could also be databases being dismounted or a lack of disk space.

Semaphore is synchronization tool to solve critical-section problem, can be used to control access to the critical section for a process or thread. The main disadvantage (same of mutual-exclusion) is requiring busy waiting. It will create problems in a multiprogramming system, where a single CPU is shared among many processes. Busy waiting wastes CPU cycles.
Deadlock is a situation when two or more processes are waiting indefinitely for an event that can be caused by only one of the waiting processes. The implementation of a semaphore with a waiting queue may result in this situation.

The main role of edge transport server is to route inbound and outbound SMTP messages, checking them for spam and virus characteristics.

The following benefits we will have :

Reduces load on Hub Transport and Mailbox servers with the filtering of unwanted messages, such as those containing spam or viruses Adds extra layer(s) of protection to the Exchange servers located on the internal network. Prevents Internet connections directly to the Exchange servers located on the internal network.