If you’re a new business, this article doesn’t apply to you – simply create user accounts in the cloud, join your Windows 10/11 devices to Entra ID, and manage your iOS and Android devices with endpoint manager, and you’re good to go.


Most businesses, however, have investments in existing technology on-premises and need to migrate to M365. This article will cover your different options:

  • Cutover migration
  • Staged migration
  • Express hybrid migration
  • Minimal hybrid migration
  • Hybrid migration
  • PST-based migration
  • IMAP migration
  • Third-party tools

Suppose you don’t have Exchange on-premises, i.e., using Lotus Notes / Domino, another email system, Google Workspace, or another cloud email solution. In that case, you’re looking at either an IMAP migration or third-party migration services.

Most of the other migration methods rely on directory synchronization, where your on-premises AD accounts are synched to Azure AD. If you’re still on Exchange 2007, 2010, or 2013 (all no longer supported), a Staged setup allows you to migrate mailboxes in batches once you’ve configured directory synchronization.

Be aware that you must manually reconfigure each user’s Outlook profile to point to O365 when their mailbox has been migrated. For smaller environments, the Cutover approach is the easiest. Microsoft talks about this method for less than 2000 mailboxes (Exchange 2003+), but in the real world, it’s probably appropriate for 100-150 mailboxes or so, depending on internet bandwidth.

The idea is to move everyone’s mailbox from on-premises to the cloud over a weekend or other appropriate downtime. If you’re on Exchange 2010+ and plan to move all mailboxes to the cloud over a few weeks, consider the Express hybrid option. If you’re more extensive and are looking at a few months of migration time, look at the Minimal hybrid alternative.

If you have a larger environment (Exchange 2010+), you expect to be in a mixed state for an extended time, and you need the ability to move mailboxes from the cloud back to on-premises (offboarding). Consider Full Hybrid for a full breakdown of the different flavors of hybrids; see here.

The various types of hybrid provide prosperous co-existence with a unified Global Address list, sharing of Free/busy calendaring information, and seamless mailbox moves for end-users; when their mailbox has been moved, they’re just prompted to restart Outlook.

If you need to keep an Exchange server (or several) around on-premises, be aware of the need to keep it up to date so as not to be throttled, and if possible, look to retire it instead using PowerShell cmdlets to manage Exchange attributes in AD.

Microsoft’s documentation will point you to the mail migration advisor, which may lead you to the Hybrid Configuration Wizard (HCW), depending on your choices in the advisor. HCW will step you through the individual steps you must take, depending on your route, including the hybrid flavors and Staged and Cutover.

IMAP migrations let you move from non-exchange systems that support IMAP with a limit of 500,000 objects per mailbox and a maximum email size of 35 MB.

If you have PST files with email on-premises, you can migrate them to Office 365; there’s even a PST Collection tool to track them down on your network and collect them. If you have lots of them, you can even ship them to Microsoft on disk.

Once you have completed your migration, you’ll need to consider your Mail Exchanger (MX) DNS record, which will have been pointing to your on-premises mail server and now needs to be changed to Exchange Online instead.

You also need to revisit your Autodiscover DNS records, which is how Outlook and other email clients find the correct Exchange server automatically.

To properly protect your Microsoft 365 environment, use Hornetsecurity one-of-a-kind services:

To keep up with the latest Microsoft 365 articles and practices, visit our Hornetsecurity blog now.


If you’re looking for a simple mailbox migration experience, Hornetsecurity offers the Mailbox Migration Tool (MMT) as part of 365 Total Protection Enterprise / Enterprise Backup.


What is Office / Microsoft 365 migration?

Office 365 migration is the process of moving data, users, and settings from an on-premises environment or another cloud service to Microsoft Office 365, ensuring a seamless transition.

How do I transfer Office 365 data to another Office 365 tenant?

Transfer data between Office 365 tenants using third-party migration tools or Microsoft’s native methods like mailbox migration or hybrid deployment, ensuring data integrity and user continuity.

How do I migrate SharePoint Server data to Office 365?

Microsoft has a free tool, the SharePoint Migration Tool (SPMT), if that doesn’t fulfill your requirements there are many third-party migration services on the market.