It looks like it’s rejected by my box when sending to that alias from other email providers.
Error Details
Reported error: 550 5.7.367 Remote server returned not permitted to relay -> 554 5.7.1 #!@zzz.com: Relay access denied
DSN generated by: DM5PR01MB2492.prod.exchangelabs.com
Remote server: box.zzz.com