In May 2023, Microsoft started to introduce the new version of Outlook for Office 365 subscriptions.
This has currently been made available to the release preview ring of Office 365, and can be enabled by end users if you have not specifically blocked this through Exchange Online Powershell or Group Policy configuration.
One Outlook consolidates a number of differing Outlook codebases into a single application.
This change does not affect the Signature 365 Outlook Add-in, or perpetually licenced versions of Office (2016 / 2019 / 2021)
This version of Outlook now uses roaming signatures for users signatures, storing these on Office 365. As signatures are no longer stored locally on the machine, the Email Signature Manager Agent is unable to set signatures if you are using the updated version of Outlook 365.
The new version of Outlook 365 also prevents the use of COM addins, although this feature does not affect the Email Signature Manager Agent.
Currently the only solution to apply signatures for Outlook 365 installations once the application is forced to update by Microsoft is to migrate to our cloud solution, Signature 365. Microsoft have currently given no indication with regard to a rollout date to the release version of Office 365.
Before the updated version is released, it is possible to use either the Office Deployment Tool or Group Policy to define a version for you Office 365 clients to update to.
This can be done using the TargetVersion update setting to prevent Office 365 apps from updating past a particular version. Please see the following article on how to achieve this.
Configure update settings for Microsoft 365 Apps - Deploy Office | Microsoft Learn
It is also possible to disable the Try the New Outlook button within the Outlook app itself in a number of ways.
Disabling access to new Outlook via Exchange Online Powershell.
Using Group Policy to assign the following registry key to your users.
Version 2304 (Build 16327.20247 Click to Run) is currently known to function with the locally stored version of signatures. This article will be updated with the last known version of Office 365 to allow this functionality.