Heavy Network Traffic Between Exchange03 Front-end Server And Back-end Server

My company has two sites setup connected through a VPN and we have no problem communicating with devices on either side of the VPN tunnel. Site A has an Exchange 2003 server configured which actually contains the mailboxes. At Site B we have an Exchange 2003 server configured as a front end server, all email traffic comes into this server and gets passed out to the corresponding exchange servers. The problem is, the exchange server with the mail boxes at Site A is sending data to the front end server and is using all available bandwidth. We have another site, Site C configured exactly the same as Site A, but it's not having any such problems. I restarted the HTTP virtual server via ESM, the netwok traffic between two sites return to normal right away, but later the problem will starts up again. I don't know if it's due to too many users' smartphone connecting exchange serve via HTTP protocol. (i can find many HTTP connections established between two server with command 'netstat -ano | findstr 80') Has any else experienced a similar problem or know where to start looking to solve this problem?Thanks for any help.