DL Security - Prevent users from making changes

[Chris4RCSD]

I have been having problems with the DL's membership changing. Then user's call up and complain that they aren't getting the email they want. I would check the DL's and find that they had been modified. I'm the only administrator to our Exchange 5.5 server and I wasn't the one making the error's.
Well I decided since this last change to check something's out and it appears that a user logged into Outlook 2000 can go to a property of DL and choose 'Modify' and do anything they want to the DL's. I'm glad to find the problem but I can't seem to resolve it.
I went into the Exchange Admin program and looked at the Permission's tab on the DL's and it says:

BUILTIN/Administrators Admin Site:xxx
NT AUTHORTIY/NETW... Admin Site:xxx
NT AUTHORITY/SYSTEM Admin Site:xxx
DDDD/Exchange Adminis Admin Receipient Container
DDDD/Exchange Adminis SAAdmin Site:xxx

(Where XXXX is the Site Name that the only server is in and the DDDD is the Domain Name)

There is currently no owner of the DL. I listed myself as the owner but that didn't seem to make a difference.

Any help would be greatly appreciated

Thanks,

Chris