Hello,
With a situation like that you will have to call them and have them test the lines.
But there are other factors to consider.
1. How far away you are from the exchange server
2. If the line is shared by people around you.
3. What time of day you are getting these speeds.
For 1. if you are to far away from the exchange servers you speed will not hit the 5mb/s cause of the fact of the distance it has to travel to get to you. Which would result in lost speed.
2. If the connection is shared that means that everyone around you that use the same provider are also using the same line to transfer data and would also slow you down.
3. If you are using and testing this at prime times like mid day or early evening then that would be a factor as most people are home from work and such and could be on their PC's doing things as well. Which would also contribute to the slow down.
All of these thigns play a factor to why you wont get the max. Even on a 5mb/s line when i am close to the exchange servers i still never see 4mb. At most i get 2.5 maybe 3 if it is early morning and not many people are on near me.
But call them and see what they say.
Cheers,
Mak