Delay in mail delivery

Mail being collected from ISP comes into SERVER OK.
The problem is that it is not getting delivered to to the reciepients quickly enough,(delays are around 2 hrs) it was OK, there has been no changes made to the server in months.
Workstations using WIN 2K, Win 95, Win 98
NT 4 Server SP 6a, running exchnage Server 5
Can anyone make any suggestions