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Exchange Server Overview and Version Planning

Why On-premises Exchange is Still Needed?​

Despite Microsoft's push for cloud adoption, on-premises Exchange Server remains necessary in the following scenarios:

1. Hybrid Deployment​

  • Exchange Server is a required architectural component for hybrid deployments
  • Managing certain M365 attributes (such as custom attributes, address lists) still requires on-premises Exchange
  • Provides local control point for mail flow

2. Regulatory and Compliance Requirements​

  • Certain industries (finance, healthcare, government) require mail to remain on-premises
  • Data sovereignty considerations
  • Audit and eDiscovery requirements

3. System Integration​

  • Integration with legacy systems (ERP, CRM)
  • Internal applications relying on MAPI/EWS connections
  • Special transport rules or mail processing requirements

Exchange Server Version Strategy​

Supported Versions (2025 Status)​

VersionLatest CUMainstream Support EndExtended Support End
Exchange 2019CU142024/01/092025/10/14
Exchange 2016CU232020/10/132025/10/14
Exchange 2013CU232018/04/10Ended
Important Note

Exchange 2013 extended support ended in April 2023 and no longer receives security updates. Upgrading is strongly recommended.

CU (Cumulative Update) Strategy​

Best Practice: Stay on latest or second-latest CU

# Check current Exchange version
Get-ExchangeServer | Format-List Name, Edition, AdminDisplayVersion

# Example output:
# Name              : EX01
# Edition           : Enterprise
# AdminDisplayVersion : Version 15.2 (Build 1544.4)

Why timely CU updates are needed?

  • Security Updates (SU) typically only support the latest two CUs
  • New features and performance improvements
  • Hybrid deployment new features require latest CU support

Exchange Architecture Fundamentals​

Basic Roles​

Exchange 2016/2019 simplified to two roles:

  1. Mailbox Role: Mailbox databases, Client Access Services (CAS)
  2. Edge Transport Role (optional): Perimeter network mail gateway

High Availability​

Database Availability Group (DAG)

Recommended Configuration:

  • At least 3 Mailbox Servers (odd number to avoid Split-Brain)
  • Witness Server located in a different site
  • At least 3 copies per Database

Hybrid Deployment Architecture​

Typical Hybrid Topology​

Key Components​

  1. Hybrid Configuration Wizard (HCW)

    • Automatically configures Send/Receive Connectors
    • Creates OAuth trust relationship
    • Configures Organization Relationship

  2. Exchange Web Services (EWS)

    • Calendar free/busy lookup
    • MailTips
    • Message tracking

  3. Mailbox Replication Service (MRS)

    • Mailbox migration (On-prem ↔ Cloud)
    • Supports batch migration

Capacity Planning​

Mailbox Server Specification Recommendations​

User CountCPURAMIOPS (per DB)
< 5008 vCPU32 GB0.1-0.2
500-200016 vCPU64 GB0.08-0.15
> 200024+ vCPU96+ GB0.05-0.10
Important Note

These are baseline recommendations. Actual requirements should be adjusted based on user behavior (mail volume, attachment size, mobile device count).

Disk I/O Calculation​

Total IOPS = (User Count × IOPS per User) ÷ Number of Database Copies

Example:

  • 1000 users
  • 0.1 IOPS per user
  • 3 Database copies
  • Total requirement: (1000 × 0.1) ÷ 3 = 33.3 IOPS

Next Steps​