Old environment. Windows 2008 R2 Active Directory forest with a child domain. Exchange 2007 in the child domain. New environment Windows 2016 Active Directory forest with a child domain. Exchange 2016 in the child domain. All AD accounts migrated and are synchronizing. All mailboxes mailbox-enabled in the new domain. Test account email is migrating fine, but when we try to switch the mailbox to the new Exchange server we get the error in the subject. We've checked for blank X.500 email addresses and duplicated values in the Adam database. We are pretty much stuck at this point as Quest support hasn't been helpful.