Microsoft Exchange Server Subscription Edition (SE) CU

Question

Microsoft Exchange Server Subscription Edition (SE) CU

Glenn Maxwell 14,206

Hi All,

I am using Exchange Server Edition (SE) Hybrid Environment. All users are created in onprem and migrated to online. I am currently running Microsoft Exchange Server Subscription Edition (SE) RTM with the February 2026 Security Update (Feb26SU). Can I directly upgrade/update from Exchange Server SE RTM Feb26SU to Exchange Server SE RTM Sep26SU (KB5121608)? Or is there a prerequisite version/CU/SU that I need to install first before applying the Sep26SU?Could you please confirm the supported upgrade/update path and any prerequisites I should be aware of?

[PS] C:\Windows\system32>Get-Command Exsetup.exe | fl


Name            : ExSetup.exe
CommandType     : Application
Definition      : C:\Program Files\Microsoft\Exchange Server\V15\bin\ExSetup.exe
Extension       : .exe
Path            : C:\Program Files\Microsoft\Exchange Server\V15\bin\ExSetup.exe
FileVersionInfo : File:             C:\Program Files\Microsoft\Exchange Server\V15\bin\ExSetup.exe
                  InternalName:     ExSetup.exe
                  OriginalFilename: ExSetup.exe
                  FileVersion:      15.02.2562.037
                  FileDescription:
                  Product:          Microsoft® Exchange
                  ProductVersion:   15.02.2562.037
                  Debug:            False
                  Patched:          False
                  PreRelease:       False
                  PrivateBuild:     False
                  SpecialBuild:     False
                  Language:         Language Neutral

Also, my Active Directory environment has more than 600 OUs. Do I need to follow the guidance in the Microsoft article below regarding Exchange Server not displaying all OUs, or will the previous configuration/settings be retained automatically after applying the Sep26SU?

https://learn.microsoft.com/en-us/troubleshoot/exchange/administration/exchange-server-not-display-all-ou

Could you please confirm the supported update path and whether any additional steps are required for an environment with more than 600 OUs?

0 comments

Answer accepted by question author

Killian N 4,145 Independent Advisor

Hi Glenn,

Based on the build number you shared, your server is currently running Exchange Server Subscription Edition (SE) RTM with the February 2026 Security Update (build 15.2.2562.37).

From what I can see, you can upgrade directly from the February 2026 Security Update to the September 2026 Security Update (KB5121608). Since Exchange Security Updates are cumulative, there is no need to install any intermediate Security Updates first, provided that your server is already on the Exchange SE RTM baseline.

Before proceeding with the installation, I would recommend taking a few standard precautions:

Confirm that there are no pending Windows restarts.
Ensure that appropriate backups have been completed in line with your organization's change management procedures.
Run the latest Exchange Health Checker before and after the update to verify server health.
Restart the server after installation if the update requests it.

Regarding your environment with more than 600 OUs, the article you referenced relates to a specific OU enumeration issue. If you previously implemented the workaround or configuration changes described in that article, those settings would normally remain in place after installing a Security Update. In general, Security Updates update Exchange components and binaries, but they do not typically revert existing Exchange or Active Directory configurations.

Consequently, if you are not currently experiencing the OU-related issue mentioned in the article, I am not aware of any additional actions that would be required solely because your Active Directory contains more than 600 OUs. That said, it would still be a good idea to verify OU visibility as part of your post-update validation checks, just to ensure everything continues to function as expected.

Once the update has been installed, you can run the Exchange Health Checker again and verify the Exchange build number to confirm that the update completed successfully and that the server remains healthy.

To learn more, please visit:

I hope this helps. As an Independent Advisor, I can share guidance based on the information available, but if you have any concerns about the update in your specific environment, it would be worth testing the update in a non-production environment first, where possible.

Kind regards,

Killian N 4,145 Reputation points Independent Advisor

2026-09-16T16:27:10.0266667+00:00

Hi Glenn,

Just checking in to see if you've reviewed the proposed answer.

Any update would be appreciated.

1 additional answer

Your answer

Killian N 4,145 Reputation points Independent Advisor

2026-09-16T16:27:10.0266667+00:00

Hi Glenn,

Just checking in to see if you've reviewed the proposed answer.

Any update would be appreciated.

Answer 1

Yes. A direct update from Exchange Server SE RTM Feb26SU (15.02.2562.037) to Exchange Server SE RTM Sep26SU (15.02.2562.049 / KB5121608) is supported.

The supported path is based on these facts:

Security Updates (SUs) are cumulative.
- Microsoft states that SUs contain all security updates since the CU they apply to was released.
Exchange SE RTM remains the same CU baseline here.
- The build list shows both Feb26SU and Sep26SU as Exchange Server SE RTM builds.
No intermediate SU is required before applying Sep26SU.
- Since both are SUs on the same SE RTM baseline and SUs are cumulative, Feb26SU can be updated directly to Sep26SU.

For the versions mentioned:

Feb26SU: 15.02.2562.037
Sep26SU: 15.02.2562.049

Prerequisites and cautions supported by the documentation:

Keep Exchange in a supported, up-to-date state.
Install the latest Windows updates on the server before Exchange updates.
Remote Registry must be set to Automatic and not Disabled.
If moving to a newer CU, note that a CU is a full installation and cannot be uninstalled to revert; uninstalling removes Exchange from the server. This caution is less severe for SUs/HUs, which operate differently and can be removed when necessary, but only after careful vetting.

For the more than 600 OUs question:

The available context does not provide official Exchange SE documentation for that specific OU display article. The only relevant context is a Microsoft Q&A answer stating that Exchange SE is code-equivalent to Exchange 2019 and that the 500-item hard limit in the ECP picker remains in effect unless manually adjusted in server configuration.

What can be confirmed from the available context:

Exchange SE RTM is code equivalent to Exchange 2019 CU15 except for the license agreement, product name, and build number.
A Microsoft Q&A answer indicates the 500-item hard limit behavior still applies in Exchange SE.

What cannot be confirmed from the available context:

Whether applying Sep26SU automatically retains or changes any prior custom setting related to that OU display limit.
Whether any additional post-update action is required specifically for the OU display article referenced.

A related Exchange SE feature that is documented is custom configuration preservation: Exchange setup now backs up and restores the most common configuration files so admins no longer need to manually restore or reapply them. That is documented for Exchange SE setup behavior, but the context does not explicitly state that the OU display-limit setting from the referenced article is included in that preserved set.

So the supported answer is:

Directly update from Feb26SU to Sep26SU.
No intermediate SE SU/CU is required for that move.
For the 600+ OUs scenario, the context supports that the 500-item picker limit still applies in SE, but it does not confirm that Sep26SU will automatically preserve or change any custom workaround for that limit.

References: