How do I consolidate multiple on-premises email ​​​​organizations to a single on-premises Destination?

 Answer:

Migration Planning is required if you are dealing with multiple mail organizations on-premises that need to be consolidated into one single organization, the same mail system (e.g., Exchange) but with different versions, or when dealing with a mixed environment (e.g., with GroupWise and Exchange).

Take the following scenario of a mixed environment as an example: 

• ​a GroupWise environment
• an Exchange 2003 Source environment
• an Exchange 2007 Destination environment

1. ​Set up your Exchange 2013 environment, and create all the mailboxes on the Destination environment. ​
2. Create a MigrationWiz account on your Exchange 2003/2007 and 2013. Click article to set up the GroupWise migration.
3. Creat​e a project in MigrationWiz with a Source of Exchange 2003+ and a Destination of Exchange 2007+ (Exchange 2003 environment).
4. Fill in the OWA URL and the administrator accounts you created.
5. Creat​e a project in MigrationWiz with a Source of Exchange 2003+ and a Destination of Exchange 2007+ (Exchange 2007 environment).
6. Fill in the OWA URL and the administrator accounts you created​.
7. Creat​e a project in MigrationWiz with a Source of GroupWise and a Destination of Exchange 2007+ (GroupWise environment).
8. Attach the mailboxes to the appropriate project and start the migration. ​​
   

You can choose to go for ​cutover, prefill, or backfill. Read more here​.