Email needs to be delivered on time. This may be a service agreement requirement, but also it is good policy to have mail delivered as quickly as possible. Slow end-to-end times could indicate a number of things. It may be that the server is not working properly, or that certain times of the day experience overwhelming message loads, or that the existing hardware resources are being pushed beyond their capacity.
Mail takes a longer period of time to be delivered than normal.
Use any facility that sends a message and receives it. Compare the headers times between server hops, and times between point of origin and retrieval. See 27.8.1 immonitor-access.