Joplin doesn't save this information for OneDrive. Joplin actually doesn't know that. The only way to receive this information requires developer skills:
When you log in to OneDrive Joplin receives a refresh and an access Token which allows Joplin to interact with the OneDrive Api. I think you could get the email address by making a http request to the OneDrive Api using the refresh Token which is saved in the DB (I think you basically would need to get an updated Access Token first). The refresh Token is saved in the table settings inside the JSON Value for the key sync.3.context.
If you want to go on, you might find this information useful.