You can use a public folder to store and share global email contacts, but a better approach is to use Exchange mail contacts. These objects show up in the Exchange GAL and OAB and are available to all Outlook clients (and some third-party clients too).
Microsoft has refreshed the Outlook Mobile architecture (now called “Microsoft Sync Technology”). They suggest that you run some PowerShell to report clients connecting via the old and new architectures. Their code works, but we think ours is better.
Microsoft has published some statements about not allowing older Office clients to connect to Office 365 from October 2020. Well, the word “allow” should really be read as “support.” Or so we hear.