Hadn't heard of Notify or EmailResource - what great plugins.
I'm not handling user registration. I just have a public contact form on one of my dev sites that needs to return the information back to the site owner. Changing the core email settings via the manger in MODX was enough to get this working
I guess I could setup each client with a Mandrill account, or just use the one I setup. The outbound email quote is very generous, especially when you're using it for a contact form!