Signature 365 Classic Agent cannot support Microsoft One Outlook

Background

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.

Cause

The new version of Outlook now uses roaming signatures for user's signatures, storing these on Office 365. As signatures are no longer stored locally on the machine, the classic 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 classic agent.

Solution

Currently, the only solution to apply signatures for Outlook 365 installations once the application is forced to update by Microsoft is to use the Signature 365 Modern Add-in. Microsoft have currently given no indication with regard to a rollout date for the updated 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 your 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.

https://learn.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/outlook-on-the-web/enable-disable-employee-access-new-outlook

Using Group Policy to assign the following registry key to your users.