I have a customer whose domain is not currently hosted by us. They have to change their outgoing SMTP server address when they want to send emails from other places as they travel. I would like to be able to offer them SMTP authentication if that is possible. Could I set up a sub-domain (e.g. relay.mydomain.com), set up an email address with any username and password and give this to them to use in the settings for "My Server Requires Authentication"? Grateful for any view on how best this could be done.