Before you reattach a mail box be sure that the retention for keeping deleted mailboxes is greater then 0 days, to do this right click your Infromation store from Exchange System Manager properties then in the Limits tab ensure there is a value greater than 0 in the “Keep deleted mailboxes for (days)” field. You should find the default is greater than 0 days anyway.

Detach an Exchange user mailbox
To de-attach mailbox go into Active Directory Users & Comptuers find and right click properties on the user’s account, from here select “Exchange Tasks” then either select “Delete Mailbox” or “Remove Exchange Attributes” wait for recipient update services (RUS) to run or run RUS manually.

Re-Attach an Exchange users mailbox
To attach the mailbox to the new user go into Exchange System Manager find the mailbox under mailboxes in the information store (you will notice the de-attached mailbox has a red circle with a cross thru it) then select reconnect, now specify the new account (ie Domainusername or username@domainname.com). after RUS has run the mailbox should be connected to the new account.

Notes
You cannot reattach a mailbox to a user that already has a mailbox, for example if you deleted a user from Active Directory then recreated the user he/she might have 2 mailboxes shown in Exchange, you must detach the user from the current mailbox before you can re-attache them to the old mailbox or another mailbox.

Article written by MyComputerAid.com

Post from: MyComputerAid.com Computer support

Reattach deleted users exchange mailbox