IT Server Support

I Manage an MS Exchnage 2003 server that acts as a post office for 6 different branches throughout the country.I currently have 2 leaselines to 2 branches that receive emails from this mail server but are currently so slow to download emails via the leaselines.Branch A has 128kb and branch B has 65K.

Can someone please explain in details how I can have all replications to each sites be done at an instance or rather much faster.