Cross-Forest Exchange to Exchange Migration: Step-by-Step Guide

Email systems are piling up exponentially. Companies do mergers and acquisition, organizations/teams split, or infrastructure decision changes over time. One common result is multiple Microsoft Exchange environments running in different Active Directory forests.

No matter what the reasons are, when many users need to move from one forest to another, a cross forest Exchange migration becomes necessary. However, it is a process to migrate the Exchange mailbox and other objects from one Active Directory to another.

Since, this is a tedious task and often users tend to miss out the correct process; we’ve come up with guidance. This guide will assist you in the Exchange cross forest migration step by step. The main objective of this article is to help you understand what happens during an Exchange to Exchange migration, why each step matters, and what to take care of.

Easy Steps for Cross Forest Exchange Migration

Know and follow the below complete step-by-step guidance for an error-free cross forest Exchange migration:

1. Review Both Exchange Environments
2. Prepare Users in the Target Forest
3. Manage Network and Permission Access
4. Choose Migration Method
5. Do a Trial Cross Forest Exchange Migration
6. Perform Exchange to Exchange Migration
7. Clean up the Source Environment

Let's go with each step in detail:

Step 1. Review Both Exchange Environments

Firstly, assess what you're going to migrate. Also, analyze the Exchange versions in both of the forests, know the number of users, mailboxes, shared mailboxes, and resource mailboxes. Also, documentize email address forms and format along with mail follow rules & connectors. These practices create a clear baseline to counter any further conflicts.

Step 2. Prepare Users in the Target Forest

Since a mailbox can not exist without users, create the user account and match the user via email address and unique IDs in the target forest. Also, enable the users’ mail so that they can receive messages. When you have accurate user mapping, you prevent duplicate accounts and login confusion post migration.

Step 3. Manage Network and Permission Access

Cross forest Exchange migration needs temporary permissions & access between different environments. Establish the network connectivity between the source and destination Exchange server. You may also need limited permissions to read the mailbox data. But make sure your security team verifies this step prior to migration.

Step 4. Choose Migration Method

The effective cross forest exchange migration depends on a reliable migration method. You can use Microsoft's native Active Directory Migration Tool (ADMT) to perform cross-forest migration. It simplifies the migration process and structures tasks in Active Directory Domain Services (AD DS). With this tool, you can migrate users, groups, etc., between AD DS domains in different forests (inter-forest) or between AD DS domains in the same forest (intra-forest).

Besides, you can also perform cross forest migration using the PowerShell Cmdlets. However, you need to prepare the source and destination before starting the migration, so prepare mailboxes for cross forest moves in Exchange Server.

Step 5. Do a Trial Cross Forest Exchange Migration

Never migrate all user mailboxes at once. It may create severe data loss issues. For this, start migration with a small group, containing different departments, small and large mailboxes, aligning with the users and their calendars, and sharing calendars.

Once you check email delivery, email platform behaviour, calendar access, synchronization, etc., do a full migration if it looks good and reliable.

Step 6. Exchange to Exchange Migration

Post a successful cross forest Exchange migration, migrate data in small batches. You must make sure to copy the mailbox to the destination and the source mailboxes remain active until every bit of data is migrated.

You can even schedule the migration during off-hours. After migration, you must check each batch by keeping a check on the mailbox size, try to send & receive emails, review calendar access, etc.

Step 7. Delete the Source Environments Data

After you are satisfied with the full migration procedure, remove old connectors and disable unused mail enabled users along with the temporary permissions. With this, you will counter any confusion in both source and destination Exchange environments.

Last Say | Cross Forest Exchange Migration

A cross forest Exchange migration works best when you are familiar with the process and working steps. The above-mentioned steps, like source analysis, migration methods, pilot test, etc., make the process operational and keep the user connected throughout the migration process.

When planned as guided, the Exchange to Exchange Online migration becomes a hassle-free transition, instead of a disturbing one. So, prepare a plan, inform your users, and do a migration if needed.