✨(mailboxes) cannot create mailbox with same local part as alias

should not be able to create a mailbox having the same local part as an alias

src/backend/mailbox_manager/api/client/serializers.py

@@ -78,6 +78,17 @@ class MailboxSerializer(serializers.ModelSerializer):
 
         return mailbox
 
+    def validate_local_part(self, value):
+        """Validate this local part does not match a mailbox."""
+        if models.Alias.objects.filter(
+            local_part=value, domain__slug=self.context["domain_slug"]
+        ):
+            raise exceptions.ValidationError(
+                f'Local part "{value}" already used by an alias.'
+            )
+
+        return value
+
 
 class MailboxUpdateSerializer(MailboxSerializer):
     """A more restrictive serializer when updating mailboxes"""
@@ -343,7 +354,7 @@ class AliasSerializer(serializers.ModelSerializer):
             local_part=value, domain__slug=self.context["domain_slug"]
         ).exists():
             raise exceptions.ValidationError(
-                f'Local part "{value}" already used for a mailbox.'
+                f'Local part "{value}" already used by a mailbox.'
             )
 
         return value