mass email delivery to local users

[tiger2000]

Dear All,

Recently, we install a 5.0.13 Open source ZCS and there are about 18000 accounts on this system for testing.

We found when we send an email (6K size) to local users (18000 users) , it almosts takes 30 minutes to complete this action.

Is this normal hehavior , or is there anything we can tune the system to gain more performance?

Thanks.