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Cumulative Update 21 for Exchange Server 2016

Microsoft released Cumulative Update 21 for Exchange Server 2016 (KB5003611) on June 25, 2021. Also known as Exchange Server 2016 CU21. This Cumulative Update includes fixes for nonsecurity issues and all previously released fixes for security and nonsecurity issues. These fixes will also be included in later Cumulative Updates for Exchange Server 2016.

How to update Exchange Server 2016 to CU21?

It’s recommended to update the Exchange Server in the lab environment before updating it in the production environment.

  1. Download Cumulative Update CU21 for Exchange Server 2016 (KB5003611)
  2. Update to Microsoft .NET Framework 4.8
  3. Install Exchange Cumulative Update

Cumulative Update CU21 for Exchange Server 2016 is released as version 15.01.2308.008. Which version are you running now? Read more on how to find your Exchange version build number.

Cumulative Update 21 for Exchange Server 2016 download

Known issues in Cumulative Update 21

In multidomain Active Directory forests in which Exchange is installed or has been prepared previously by using the /PrepareDomain option in Setup, this action must be completed after the /PrepareAD command for this Cumulative Update has been completed and the changes are replicated to all domains. Setup will try to run the /PrepareAD command during the first server installation. Installation will finish only if the user who initiated Setup has the appropriate permissions.

Issues fixed in Cumulative Update 21

  • 5004612 Message body not displayed in OWA if the message was added in Outlook to a new mailbox
  • 5004613 OutOfMemory exception when moving a public folder that has a large ICS sync state
  • 5004614 Korean text is garbled in calendar invitation to a user with a Chinese display name
  • 5004615 “InvalidOperationException” and Store Worker process crashes during mailbox move
  • 5004616 Changing the email address in EAC doesn’t work in modern browsers
  • 5004618 MSExchangeMailboxAssistants 4999 Crash in ELCAssistant.InvokeInternalAssistant with System.NullReferenceException
  • 5004619 Mailbox creation through ECP fails after installing Exchange Server 2019 or 2016 April update
  • 5004629 No version updating after you install Exchange Server 2016

File hash information

  • File name: ExchangeServer2016-x64-cu21.iso
  • SHA256 hash: 403EFE9589709461FCC09B332894C4ED1F0D93414D9DBDCED1A0967727C47063


ALI TAJRAN is a passionate IT Architect, IT Consultant, and Microsoft Certified Trainer. He started Information Technology at a very young age, and his goal is to teach and inspire others. Read more »

This Post Has 2 Comments

  1. Hi Ali, thanks for the write up.

    Any chance you could advise here?

    CU21 done on Exchange 2016, hybrid environment.

    What im finding is that EXO users with linked (full perm.) on-prem mailboxes started having issues on weekly basis.
    Outlook freezes and they are unable to do anything until we restart local exchange.

    Your help is muchly appreciated.

    cheers, Matt

    1. Hi Matt,

      Exchange Server 2016 CU21 and Exchange Server 2019 CU10 introduced AMSI (Antimalware Scan Interface) integration.

      It’s not working that great with some security products installed on the Exchange Server. Disable AMSI temporarily and check if that helps.

      Read more in the article: Outlook issues due to Exchange Server AMSI integration.

      After verifying that it’s working fine, contact your security product and ask them to update their product. After the security product is working as expected, you can remove the server override.

      I hope this helps.

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