restrict bandwidth usage

[Opesh Alkara]

Dear All,
 (OS : Red Hat Linux ES 3.0) 
 Just wanted to know how can I restrict the banwidth usage by ethernet 
interface of a particular/mulitple Linux machine in my internal LAN.
 I want to limit my Linux systems to use only 75 % of the available machine 
for any type of trafic. To be more specific at any time these systems should 
not cross the 75% of my LAN bandwidth. These machines are mass mailing 
servers and hence use up all the available LAN bandwith to transfer mails to 
a central HUB mailserver which eventually delivers all the mails. Since they 
use all the available bandwidth leaving no room for admins to login and 
monitor I wnt them to restrict to 75% of LAN bandwidth. Hope I have made my 
point clear
 Waiting for replies.
 Thanks for reading the mail. 

-- 
Regards
Opesh Alkara