Well you did mention that some of your customers get a routing error when trying to send you emails. That to me points at either the DNS is not setup properly or its not refreshing fast enough after your ip address changes.
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I'm also getting some errors from some of my customers. They get a rouing error when they try to send me emails.
I would say that at the moment they you will have to be patient with the isp while they grow. Let then know that you want a static IP as soon as one becomes available.