Currently we have an empty root forest with child domains. One of these child domains contains our current Exchange 2003 organization. In our organization we make use of distribution groups (mostly global) and mail enable global security groups. When installing Exchange 2007 into the root forest, will it break these mail enabled security groups? Where can I find information that this will not happen or that it will happen?

Most likely you will hit mail disappear issue while sending mails to global groups because limitation of global group is, it can be replicated across the domain but membership is replicated to local DCs/GCs only so when you install Exchange 2007 in another domain and users try to send email to global group of another domain, membership will not be emulated and mails will not be delivered and no NDRs will be generated since Exchange thinks that there isn't any member in this group based on local DC/GC.... Below KB describe you behavior of various groups.... Troubleshooting mail transport and distribution groups in Exchange 2000 Server and in Exchange Server 2003 http://support.microsoft.com/kb/839949/en-us You can refer below article to convert those from global to universal... Need to convert Global groups to Universal groups? Do you have messages to global groups disappearing? http://blogs.technet.com/sfodel/archive/2008/08/04/need-to-convert-global-groups-to-universal-groups.aspx Amit Tank | MVP Exchange Server | MCITP: EMA, MCSA: M | http://ExchangeShare.WordPress.com