emails sent to @mac.com domain not received in icloud account

All emails sent to the "@mac.com" alias in my icloud account are not being received. Mail sent to the @icloud.com and @me.com domains are being received. All outgoing mail is being sent (and it is being listed as being sent from the "@mac.com" domain unless I send I message to myself using the @mac.com domain.

In the preferences, all three domains are listed as alias's. This seems to have started on Jan 1, 2021.

No preferences or settings have changed in the mail app or on the server icloud.

Anybody have an idea how to solve this issue?