Symptoms
Using the AlwaysUnverified option for the AllowedDomains setting unexpectedly treats the domain of an incoming message as "discovered" in Skype for Business Server 2019. You expect that this option will require that messages are allowed only for domains that are specified by having the sender and recipient add each other to their Contacts list.
Resolution
To fix this issue, install the December 2021 cumulative update 6 for Skype for Business Server 2019, Core Components.