Since Microsoft released “Roaming Signatures” it has been expected that there will come support for this in the Graph API, so that external applications, for instance IAM systems, can update signatures on behalf of all users in a company.
Here is link to a discussion regarding this.
Here is link to an inquiry from the feedback portal.
From the discussion where it says that the support for this is on the roadmap, however no one can say when it will be released.
We are in desperate need for this API-support as the application we use now for managing signatures has EOL later this year, and it does not support Exchange Online due to this lack of API-support. Our organization (approx. 8000+ users) will then loose the managed signatures. This is particularly bad since we are facing a transition to the New Outlook.
We hope sincerely that Microsoft will prioritize this and release support for Roaming Signatures within the Graph API very soon.
