Location of Mail.Queue file D:\Program Files\Microsoft\Exchange Server\TransportRoles\data\Queue
We have done some research and findout that few of the outlook clients are below outlook 2003 SP2. This is the know issue that mail.queue file will increase if you have below outlook 2003 SP2 clients. We upgraded all the outlook clients in to Sp2 higher.
After done this also problem didn’t resolve.
Then we rename old mail.queue file ( before this we stopped Exchange trasnport service and rename the file). Then start the service. It creates a new mail.queue file in the same location.
So the proble solved temporarly. We are monitoring this file every day. I will update you if we come across the same problem