We have 2 forests with Exchange environment. The forest A is installed Exchange 2010 server and the forest B is installed Exchange 2003. We are planning retire the Exchange 2003 server in forest B; and re-create all mailboxes in the Exchange 2010 server in the forest A. Suppose we can grant permission in forest A for allow forest B user to access the Exchange 2010 mailbox by the existing user A/C in forest B. But no idea how to grant it. Kindly advice or share your experience to me. Thanks AA

Presuming there is a trust, you simply grant the trusted account full mailbox and send as permissions. Create the accounts on Exchange 2010 as Linked Mailboxes - all documented on Technet. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

I tested it but cannot work. Still pop up login box in client side. Can you share this MS KB to me? Thanks!!

How long did you wait? Exchange caches permissions, therefore a change will not be effective immediately. It can take two hours for a change to be effective. Simon.Simon Butler, Exchange MVP Blog | Exchange Resources

Hi, You can know more form these links: Create a Linked Mailbox http://technet.microsoft.com/en-us/library/bb123524.aspx How to implement Exchange Server 2007 in an environment with multiple forests where all mailboxes will be hosted in a Resource Forest(This is for Exchange 2007, in Exchange 2010 there may some differences) http://www.msexchange.org/articles_tutorials/exchange-server-2007/planning-architecture/deploying-exchange-resource-forest-part1.html http://www.msexchange.org/articles_tutorials/exchange-server-2007/planning-architecture/deploying-exchange-resource-forest-part2.html If anything is unclear, please feel free to let me know. Best Regards, Evan Please remember to click “Mark as Answer” on the post that helps you, and to click “Unmark as Answer” if a marked post does not actually answer your question. This can be beneficial to other community members reading the thread.