Undisclosed? That may be the problem. It should come up as "good" if you're not on one of their lists, I think. You may need to contact AOL and see what the problem is.
I'm sorry that the Easy SMTP caused yet another issue to diagnose. There is a "Testing and Debugging" settings section in Settings -> Easy WP SMTP that you can use to send a test message to make sure your SMTP setup isn't the issue with the failed send.
One other thing to check if you haven't already is to be certain the emails aren't going into a spam folder on your clients' AOL accounts. The clients I've had that have clung to AOL were the type that didn't think to check their spam folder when they "didn't receive" an email.