Limiting allowed domains for mail server

[chiuchemandli]

Hi

since I'm running a domain for development teams and I want to use this domain as dev mail server i see a need to restrict the possibilities for sending mails.
Fact is:
- the domain is up and running
- mail and webmail is up and running

right now I'm searching for a solution that only inbetween this domain email were exchanged.
If the domain would be www.admin.ch all @admin.ch mail accounts are restricted to send and receive mails from @admin.ch.

Do you understand what I mean and have you any suggestion for me?

Thanks in advance for the fish - best regards, chiuchemandli