Infrastructure Briefing — October 14, 2025

Executive briefing: Microsoft’s lifecycle fact sheet lists 14 October 2025 as the end of extended support for Exchange Server 2016. Security fixes and daylight-saving updates will stop, and hybrid environments will lose a supported on-premises bridge unless upgraded. Organizations should complete migrations to Exchange Online or move remaining on-premises workloads to Exchange Server 2019 or the Subscription Edition.

Key risk themes

• Unpatched mail infrastructure. Post-cutoff CVEs affecting Outlook Web Access, EWS, or transport will not be fixed for 2016.
• Hybrid supportability. Unsupported hybrid servers can complicate Autodiscover, OAuth, and hybrid modern auth configurations.
• Compliance and audit. Regulators and auditors expect supported messaging platforms; remaining on 2016 can trigger findings.

Operational priorities

• Migration sequencing. Prioritize mailbox moves to Exchange Online, decommission legacy DAG members, and refresh load balancer and certificate configurations.
• Server replacement. For on-prem estates, plan Exchange Server 2019 or Subscription Edition deployment with supported Windows Server baselines.
• Security hardening. Remove legacy auth endpoints, patch to latest CU, and validate backups before decommissioning.

Enablement moves

• Communicate the October 2025 cutoff to business owners and MSPs, including hybrid dependency impacts.
• Update SOPs for certificate rollover, SMTP relay, and transport rules on the target platform before final cutover.
• Schedule penetration tests or configuration reviews to validate Exchange Online security posture after migration.

Sources

• Microsoft Lifecycle — Exchange Server 2016
• Exchange Server 2019 feature overview

Zeph Tech manages Exchange modernization projects with hybrid coexistence plans, certificate hygiene, and decommissioning runbooks.

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