Hello,
What version of SEP clients are you running?
Are these managed clients?
If yes, are these clients communicating to SEPM properly?
All Liveupdate servers are using akamai technology for definition and patch updates..
When you run Liveupdate it goes to -
liveupdate.symantecliveupdate.com
or
liveupdate.symantec.com
then these site search which is the closest Akamai server in your location and then from those akamai server your definitions get downloaded.
Symantec definitions are hosted on Akamai servers. So that for liveupdate around the world everybody doesn't have to go to US Liveupdate servers.
Check these Threads:
https://www-secure.symantec.com/connect/forums/akamai-technologies-and-sep-installs
https://www-secure.symantec.com/connect/forums/akamai-killing-my-bandwidth
https://www-secure.symantec.com/connect/forums/website-allow-firewall-sep-liveupdate
Hope that helps!!