My users have serious problems with my server zimbra version opensource 5.0. the connection is slow. and my users have problems with me, because the client thunderbird is slow to connect with my server. this server have 2 ips (an internal and other external) my dns internal is windows 2000 server and my lan is windows 2000 server with active directory. My zimbra have 2 ips. and external my server is

This server have 501 accounts, and my ram is 1.0GB and xeon processor of 1.8MHZ. I am analizyng my lan and my dns because also I have problems with resolving.

PLS i need your ideas, because zimbra is a excellent solution but my clients have problems and are very angry, thunderbird work very good in external (wan user) but a user lan is where i have problem.