Move mailboxes accross forests

Yes, you would need to use ADMT. After migrating the users then You can move the mailboxes using the Exchange Management Shell.
Read the article below for the move-mailbox command
http://technet.microsoft.com/en-us/library/aa997145.aspx
 

This should help in your case

http://msexchangeteam.com/archive/2006/11/02/430289.aspx
http://technet.microsoft.com/en-us/library/aa997145.aspx
http://exchangeserverinfo.com/2007/12/18/transition-from-single-forest-to-crossforest-user-account-and-mailbox-movement-from-one-forest-to-another-forest--part4.aspx

- Dream

Any updates?

Good articles here! However, I dont think a mailbox cross forest move will work for me. I am not planning on using the ADMT tool to migrate Sid history over. This is an acquisition scenario. Company X has been acquisitioned and needs to be migrated to company Y. However, Company x users already have new AD accounts setup in company Y domain (different user logon naming conventions).  We setup accounts for company X users and create mailboxes that forward mail to company x mail server. This way Company Y users see company X users in the gal and we can create company Y domain name space smtp addresses for company X users o start receiving mail with the new company email address.

I know there is probably a better way to do this with ADMT. I hope to implement it in my acquisitions in the future.

But for now I need to move or merge company X exch2007 mailboxes to company Y exch2007 server. From what I read I cannot do this without using ADMT and moving the SID history out???

I believe my only option is to export company X mailbox data out of their mail server and into company Y mail server right ?? A move wont work for me ?  

You are right on track, move does not seems to an option in your case, if users are logging in using different credentials then the best option would be to Export the data and merge it with the new company's mailboxes..

- Saakar

Are we positive I cannot move the mailboxes across the forest domain x (without the sid history) to the new exch2007 domain y  and have them show up as offline? Then associate the users account to that mailbox and bring it online?