Outlook del/add messages is slow

When new mail arrives in the users inbox it appears almost animated and you can actually see the messages move down to allow new mail to appear. The same occurs when you delete a message, the other messages in the inbox move up to fill the space.This is the only thing that is affected as everything else runs at normal speed.

Also when the user logs on to another PC the problem does not occour, however swapping the PC with another on the same network cable does not solve the problem.

Any ideas as to the cause of this