First off I am not an exchange admin (so bare that in mind with your responses). But, say user X leaves the company and user X had a mailbox on your exchange server. I beleive admins will disable his domain account in active directory, but what happens with external client Y emails into his mailbox, does disabling them in AD mean they will get some sort of bounce back email saying user X has left, or is there some more work to be done by the exchange email to ensure clients arent emailing a dead mailbox and not knowing its a dead mailbox? Can you explain the process?

The short answer is no; the mailbox still exists and can receive mail. With the proper permissions, other users can connect to the mailbox (think resource mailbox in Exchange 2003). What do you want to do with the mailbox? Do you want it to exist, continue to receive mail, and be accessable by other users? Do you simply want mail sent to the old address redirected somewhere else? Do you want to disable the mailbox completely, but keep the content around so it can be accessed later? In Exchange 2010, for the last scenario you would need to use disable-mailbox. http://technet.microsoft.com/en-us/library/aa997210.aspx John

trigger OoO?Thank you, msdn =) 99.9% of my questions have been answered :D

Not going into in depth, there will be no bounce back unless the mailbox has been deleted. Then they will receive a standard NDR saying the receipient deosnt exist. If you want to let external users know that users has left, you can setup a rule for to send a autoresposne for every emal sent to the leavers mailbox. Sukh