Exchange Server Time

I have Exchange 5.5 Running on NT 4.0 SP6. I have a facility in Mississippi and Tennessee. The Tennessee site is where my Exchange server is and it handles email for both facilities. Both sites are in the same time zone. The problem that I am having is that a user in MS can send an email to a user in TN and when the user in TN replies it shows that they replied about 1 hr before the user in MS sent the original email. The time on the server is correct and the time on the PCs are the same. Also the sites log into different domains. The TN site is NT PDC and the MS site is a W2K AD domain. But the time on the servers are all the same.Does anybody have any idea where I can start looking? Thanks