The linked mailboxes seem to work - my next question is when I link the mailbox to a user in another forest - does that mailbox inherit the users username/password or does the user have to use the new password? Also - can i auto populate the server from the other forests?

The mailbox does not have a password...ever. The mailbox is just mail storage tied to an AD account. It doesn't matter where that AD account is, the AD account holds the password. Generally with multi-forest configs, you created a place holder AD account in your mail hosting forest, and then use the mailbox permissions on that place holder account to grant the external forest account permissions to it. In most cases the place holder account in the mail hosting forest is disabled. So in that scenario there are two passwords in use, one for the disabled place holder account, and the other for the associated external account in the other forest. Does that answer your question?

this was perfect - thank you!

I cannot seem to find the original posting of this But is there a KB for "multi-forest configs, you created a place holder AD account in your mail hosting forest, and then use the mailbox permissions on that place holder account to grant the external forest account permissions to it. In most cases the place holder account in the mail hosting forest is disabled. So in that scenario there are two passwords in use, one for the disabled place holder account, and the other for the associated external account in the other forest."