Making certain users invisible

[phulden]

Due to spam problems I'd like some users of my Linux system to appear "invisible" from the outside world. The problem is that I'd like some users be reachable externally.

This is what I'd like to accomplish:
I have user1@domain1.com, user2@domain1.com, virtualuser@domain2.com

user1 is a logon account which should not be able to send mail to unless from localhost or my private LAN.
user1 gets mail delivered using fetchmail from an external POP3-server.

user2 shall be reachable externally

virtualuser@domain2.com shall be reachable externally

How do I setup this? Can I enable/disable domains on a user basis using Postfix.

My Postfix version is 2.2.5. My Kernel version is 2.6.13

 

[phulden]

Never mind,
reading the Postfix manual more carefully I could figure it out using the example of protecting mailingslist under the relay section.