Спасибо за наводку. Помогло так:Delete the Discovery Search mailbox user from Active Directory. Start your Active Directory Users and Computers mmc and look for your Discovery Search Mailbox user. Default this user will be placed in the Users organizational unit. It will be named something like: DiscoverySearchMailbox{D919BA05-46A6-415f-80AD-7E09334BB852}. After deletion you can restart the installation of your Exchange Service Pack. It will not show the error anymore. To recreate the user use: setup /PrepareAD /IAcceptExchangeServerLicenseTerms