Hybrid Migration from Exchange 2013 CU23 om-prem to Microsoft 365

Question

Greetings. We have an Exchange Server 2013 CU23 on Windows Server 2012. We acquired 40 licenses for Microsoft 365, and I would like to maintain the on-premises server to communicate with other users that we don't want to migrate to the cloud. In this scenario, from my understanding, we need a hybrid migration. Question: Is this coexistence permanent?

Answer 1

Hybrid coexistence between Exchange 2013 on-premises and Exchange Online is not inherently time-limited from a technical perspective; it can be maintained as long as the environment remains supported and meets requirements.

However, planning should consider the following:

1. Support lifecycle and platform requirements
   • Exchange 2013 and Windows Server 2012 are both out of mainstream support. While a hybrid configuration can still function, this is not a long-term, recommended state.
   • For a sustainable hybrid, Microsoft recommends staying on supported Exchange and Windows Server versions.
2. Hybrid as an ongoing coexistence model
   • Hybrid deployments are explicitly designed to support long-term coexistence between on-premises Exchange and Exchange Online, including:
     • Cross-premises free/busy
     • Seamless mailbox moves
     • Unified mail flow and Outlook profile continuity
   • The article on decommissioning emphasizes that many organizations keep hybrid permanently when they still have on-premises mailboxes or need on-premises services (for example, SMTP relay, public folders, or directory synchronization).
3. When coexistence can end
   • If at some point all mailboxes are moved to Exchange Online and there is no need for directory synchronization or on-premises Exchange services, the hybrid can be decommissioned and the on-premises servers removed. For example, in the “All mailboxes and all cloud management” scenario, directory synchronization can be disabled and Exchange uninstalled once everything is cloud-only.
   • If some mailboxes or services must remain on-premises, Microsoft recommends keeping enough on-premises Exchange servers to support those functions and continuing hybrid.
4. Mixed on-premises and cloud mailboxes
   • In a scenario where some users stay on-premises and others move to Microsoft 365, hybrid is the correct approach and can be maintained as long as needed, subject to supportability of the underlying products.

Summary: Coexistence via hybrid is not forced to be temporary; it can be effectively permanent as long as supported Exchange/Windows versions are maintained and on-premises mailboxes or services remain in use. If the organization eventually becomes fully cloud-only with no need for on-premises services or directory sync, hybrid can then be decommissioned.

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References:

• How and when to decommission your on-premises Exchange servers in a hybrid deployment
• Use the Microsoft 365 and Office 365 mail migration advisor
• Decide on a migration path in Exchange Online
• Use Minimal Hybrid to quickly migrate Exchange mailboxes to Microsoft 365 or Office 365
• Scenario: Multi-organizational on-premises Exchange mailbox migration for Hosters using Microsoft Entra Connect and parallel hybrid