Deleting Mailbox-Enabled Users

If you choose to delete a mailbox-enabled user or group, the mailbox in the message store of the Exhange 200x server will not be disabled. To disable the mailbox, you should clear the following attributes in Active Directory before deleting the account:

Attributes to Clear

homeMDB,
mail,
mailNickname,
homeMTA,
legacyExchangeDN,
msExchHomeServerName,
msExchMailboxGuid,
msExchPoliciesIncluded,
proxyAddresses,
textEncodedORAddress

By clearing these 9 attributes from the user properties before deleting the user account, Active Directory will notify the Exchange 200x server that mailbox attached to this account should be disabled.