Signature 365 Classic Agent cannot support Microsoft One Outlook

Background

In May 2023, Microsoft started to introduce the new version of Outlook for Microsoft 365 subscriptions.

This has currently been made available to the release preview ring of Microsoft 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 Microsoft 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.

The new version of Outlook also prevents the use of COM addins, although this feature does not affect the classic agent.

Currently, the only solution to apply signatures for new Outlook 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 Microsoft 365 Apps.

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 Microsoft 365 Apps clients to update to.

Use the TargetVersion update setting to prevent Microsoft 365 apps from updating past a particular version. For instructions, see the following article.

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.