Restricting Login URL

[eintel]

Hi,
We have installed Zimbra in a hosted production environment and it is working a treat.

We have a number of domains on our site, and we want to restrict the users ability to login from another domains mail URL.

Form the web client if a customer visits http://mail.company1.com they can login to their own email by putting their username in as;

user@company2.com

Can I restrict the user to only being able to login to their web mail account from http://mail.company2.com/

We are using the latest production release of zimbra on a Redhat AS4 we are using LDAP external Authentication in our lab to try and get this working.

Thanks
Ben