Architecture & Solution Design

Visual representations of the primary migration paths and solution architectures for Exchange Server End of Support.

Table of contents
  1. Use Case 1: Microsoft 365 / Exchange Online (Cloud First)
    1. Key Components:
  2. Use Case 2: Hybrid Coexistence (Transitional/Long-Term)
    1. Key Components:
  3. Use Case 3: On-Premises Modernization (Exchange Server SE)
    1. Upgrade Path Diagram
    2. High Availability Architecture
    3. Key Components:
  4. Document References

Use Case 1: Microsoft 365 / Exchange Online (Cloud First)

The Cloud First strategy is recommended for organizations seeking the lowest TCO and a modern, always-up-to-date mail infrastructure. (Exchange Online documentation)

graph TD
    subgraph "On-Premises Infrastructure"
        AD[Active Directory]
        Apps[LOB Apps / Printers]
    end

    subgraph "Microsoft Cloud"
        Entra[Microsoft Entra ID]
        EXO[Exchange Online]
        EOP[Exchange Online Protection]
    end

    AD -- "Entra ID Connect" --> Entra
    Apps -- "SMTP Relay" --> EXO
    Internet((Internet)) -- "MX / SMTP" --> EOP
    EOP --> EXO
    Users[Users / Outlook] -- "Modern Auth / O365" --> EXO

Key Components:

  • Microsoft Entra ID: Provides identity synchronization and Single Sign-On (SSO) (Entra ID hybrid identity).
  • Exchange Online Protection (EOP): Handles anti-spam and anti-malware filtering as the primary entry point.
  • SMTP Relay: Local devices and applications are updated to relay directly to the cloud or via a simplified on-premises relay.

Use Case 2: Hybrid Coexistence (Transitional/Long-Term)

The Hybrid architecture is ideal for large organizations that need a phased migration or have specific compliance requirements that keep some mailboxes on-premises.

graph LR
    subgraph OnPrem["On-Premises"]
        EX19[Exchange Server 2019 / SE]
        LocalMB[(Local Mailboxes)]
    end
    subgraph M365["Microsoft 365"]
        EXO[Exchange Online]
        CloudMB[(Cloud Mailboxes)]
    end
    Internet((Internet)) -->|MX / SMTP| EX19
    Users[Users] --> EX19
    Users --> EXO
    EX19 -->|Hybrid Connector TLS| EXO
    EXO -->|Hybrid Connector TLS| EX19
    EX19 -.->|Cross-Premises Free/Busy| EXO
    EX19 --- LocalMB
    EXO --- CloudMB

Key Components:

  • Hybrid Configuration Wizard (HCW): Establishes the trust and secure mail flow between environments .
  • Shared Namespace: Users share the same @company.com domain regardless of where their mailbox is located .
  • Cross-Premises Free/Busy: Allows users to see each other’s calendar availability during the transition.

Use Case 3: On-Premises Modernization (Exchange Server SE)

The On-Premises strategy is for organizations that must maintain data sovereignty due to strict regulatory or data residency requirements. (Exchange Server SE new features)

Exchange Server SE was released June 11, 2025. Its RTM build is code-equivalent to Exchange Server 2019 CU15. Organizations running Exchange 2019 CU14 or CU15 can perform an in-place upgrade on existing hardware.

Upgrade Path Diagram 

flowchart LR
    EX2016["Exchange Server 2016\n(EOL Oct 2025)"]
    EX2019old["Exchange Server 2019\n(CU13 or older)"]
    EX2019cu["Exchange Server 2019\nCU14 / CU15"]
    EXSE["Exchange Server SE\n(Released Jun 2025)"]
    Cloud["Microsoft 365 /\nExchange Online"]

    EX2016 -->|"New server deploy\n(no in-place)"| EXSE
    EX2016 -->|"Recommended\nmigration"| Cloud
    EX2019old -->|"Update CU"| EX2019cu
    EX2019cu -->|"In-place upgrade\n(same hardware)"| EXSE
    EX2019cu -->|"Migrate"| Cloud

High Availability Architecture 

graph TD
    subgraph Primary["Primary Datacenter"]
        LB1[Load Balancer]
        EXSE1[Exchange SE Server 1\nWindows Server 2022]
        EXSE2[Exchange SE Server 2\nWindows Server 2022]
        DAG[(Database Availability Group)]
    end
    subgraph Secondary["Secondary Datacenter - DR"]
        EXSE3[Exchange SE Server 3\nPassive DR Node]
    end
    Internet((Internet)) --> LB1
    LB1 --> EXSE1
    LB1 --> EXSE2
    EXSE1 <--> DAG
    EXSE2 <--> DAG
    DAG -.->|"Continuous Replication\n(log shipping)"| EXSE3

Key Components:

  • Database Availability Group (DAG): Provides high availability and continuous replication of mailbox databases.
  • Exchange Server SE: Released June 2025; subscription-based successor to Exchange 2019 with in-place upgrade from CU14/CU15. Supports Windows Server Core, enforces TLS 1.2/1.3, and includes modern authentication via ADFS.
  • Load Balancer: Distributes client traffic across multiple servers to ensure service availability.
  • Windows Server 2022: Required OS for new Exchange SE deployments; Windows Server Core now supported for the first time.

Document References

Topic Microsoft Source
Exchange Server SE — What’s New What’s new in Exchange SE
Exchange SE architecture Exchange Server architecture
Exchange SE upgrade paths (blog) Upgrading to Exchange Server SE
Exchange SE system requirements System requirements for Exchange SE
Exchange Online documentation Exchange Online
Entra ID hybrid identity Microsoft Entra ID hybrid identity
Hybrid deployment overview Exchange Server hybrid deployments