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Exchange Online to Exchange Online Migration Between Multiple Tenants

There are companies merger or acquisitions taking over often between two Office 365 organizations. In such cases, companies need to move multiple employee mailboxes from one domain to another new domain. New email Ids are created for each user and Exchange Online to Exchange Online migration needs to be performed. Each employee gets a new email address and mailbox with the new company that has taken over the old one. All the emails are present in the new mailbox for each new user added to the primary company that has acquired another. This post describes in detail all the necessary steps to perform this migration.


Things to Consider Before Exchange Online Tenant to Tenant Migration

  • There is a merger taking place between two Office 365 tenants: source and target.
  • Company with a source domain name has acquired an organization with the target domain.
  • Mailboxes from source domain will be merged with target accounts.
  • After Office 365 tenant to tenant migration, each source account holder will have a new mailbox under the target domain name.
  • Active Directory console of source domain will be merged with target Active Directory using Active Directory Domain Services (AD DS) consolidation.


Steps to Perform Exchange Online to Exchange Online Migration

Following are the steps to move multiple mailboxes from a source Office 365 account to a target directory:

Step 1: Planning Phase

  • Purchase licenses with the new domain name for added users.
  • Increase licenses in Target Office 365 tenant to accommodate new mailboxes.
  • Use Azure AD connect to create new objects in target Active Directory Domain Services (AD DS).
  • Use AD DS tools to merge source and target Active Directory Domain Service records.
  • Verify sync between the two tenants before performing Exchange Online tenant to tenant migration.
  • Verify that all new users and groups are synced to the target tenant via directory synchronization.
  • Notify each new user for the delay in email routing.


Step 2: Preparation Phase

  • First, open the target Office 365 Admin Center.
  • Then, add the TXT or MX record of the source company’s domain name.
  • Now, this will verify the ownership of the domain performing Exchange Online to Exchange Online migration.
  • In Excel, create a CSV file containing two columns.
  • It contains two fields with source email address and destination email address.
  • Next, change the Time To Live(TTL) value in MX records to the least possible number.


Step 3: Migration Phase (Source Tenant)

  • Reconfigure primary MX record from Office 365 to a domain that is unreachable.
  • This will send emails to a waiting queue and deliver messages after Exchange Online tenant to tenant migration.
  • Remove the source email address from all objects before moving them.
  • Set the SharePoint Online public website domain back to the initial domain.
  • Remove Lync licenses from all users.
  • Set all subdomain email addresses in Office 365 source accounts to the initial domain of the company.
  • Reset all Distribution Lists and other objects to initial domain as well.
  • Remove email address of proxy accounts before performing Office 365 tenant to tenant migration.
  • Set the initial domain as the primary way to route all emails under source tenant.


Step 4: Migration Phase(Target tenant)

  • In tenant Office 365 Admin Center, set the auto-discover CNAME (internal/External) as optional.
  • Use AD FS to configure the new domain in target Active Directory Console.
  • Then, assign Exchange Online licenses to each new employee added to the target organization.
  • Next, point the source MX record to the new target Office 365 tenant.
  • Verify that email is being sent and received with the new Office 365 mailboxes.


Step 5: Finishing Exchange Online to Exchange Online Migration

  • Migrate Exchange Online email data in batches of a few months at a time.
  • Start the migration process and inform all superiors about the progress


Limitation of Manual Method

  • Lengthy Process: This can take months or days considering the large mailboxes of several old employees. The Exchange Online Tenant Migration can prove to be mundane and be tiring for many administrators.
  • Limited Support: The manual process can only move email folders in batch. Contacts and calendar items need to be moved separately for each mailbox.


Use an Automated Solution

Microsoft provides all the tools and techniques to across mailboxes from source to destination Office 365 accounts. Often some organizations look for professional solutions. They look for workarounds to make the task of Exchange Online tenant to tenant migration more direct. These are third-party software that can perform the migration of emails along with contacts and calendar items simultaneously. One such tool is the SysTools Office 365 Express Migrator. It can even reduce the transfer time by applying filters to emails and mailbox folders.



Exchange Online to Exchange Online migration is a next step after two companies merge or if one acquires the other. It requires a lot of steps and careful understanding of the complete process. It is a complex procedure and must be undertaken under the guidance of an experienced system administrator. It is highly recommended by many data migration experts to use a third-party tool like the one mentioned in this post.


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