Hello,
You can use the built-in Password Expiration Notifier Scheduled Task for this purpose. By default, it starts notifying users 7 days prior to password expiration and includes All Objects in its Activity Scope, which means that it notifies all users from all domains managed by Adaxes.
To have it notify only users from Domain A, you need to modify the Activity Scope of the task to include only objects from Domain A. For information on how to modify the Activity Scope of a Scheduled Task, see Modifying Scheduled Task Activity Scope.
Also, by default, the Scheduled Task is disabled, so, to start using it, you'll need to enable it. To do this, right-click the task, select All Tasks and then select Enable.