But I would still need to register using my work address - thus sharing it with an external service. That could still be acceptable (depending on contractual guarantees). Your cloud service will then send emails to internal users, thus it's me sharing outside the company the 1. contact addresses (which did or did not agree to above contract) and 2. email contents (up to users what they put inside). Hmm, not ideal. A MS Teams app, if you ever thought about one, would be able to use the user's own MS Graph for storage and for the contacts info, thus completely independent of external servers.