Hi Kim,
The workaround is applied in the Outlook client. In OWA, we cannot add the same domain suffix as safe senders. Moreover, we need to add each one in safe sender list. Since it cannot use the transport rule to apply to all users, we need to add these safe senders into all users manually.
I will forward your comments to our back end team regarding the synchronization issue. I will monitor this thread, and post here once I get any update.
Sorry for the inconvenience the issue caused. I hope the issue can be resolved soon.
Regards,
Johnny Zhang