exchange server in multiple domains

What is the relationship between the domains?
If they are three seperate domains then the only way that you can share Exchange is by having all the accounts in one domain. You can then use trusts to grant access to users from other domains.

Simon.

the 03 separate domains already have a trust .

Unfortunately for scdavies, you cannot compare how Exchange 5.5 and Exchange 200x operate with trusts and accounts.

Exchange 5.5 operated its own database and just used the domain for authentication. This meant operating with trusts was very simple.

With Exchange 200x it is more difficult - a mailbox must have an Active Directory account associated with it. As you are operating trusts and not child/parent you will have to create an account for every mailbox in the domain with Exchange, then configure the trusting account to have permissions on the mailbox.
Thus everyone outside of the main domain will have two accounts - an account in the main domain and an account in their own (trusted) domain.

Simon.

To Sembee: I thought if there is a trust between domains there is no need to create an account in domain1 for a user objet  that resides in domain1.
 
<<<<<Thus everyone outside of the main domain will have two accounts - an account in the main domain and an account in their own (trusted) domain. >>>>

so if I have exchange in domain 1 and have a trust to domain2 and domain3
users from domain2 and domain3 will have automatically there mailboxes in mailbox store in exchange server, of course if we mailbox-enable their accounts.