Will <Nabble> wrote
I read the raw email header, but it doesn't tell me much. I think it's probably your email server require some strict rules. I forwarded your email to our mailing list expert but he is traveling right now, so it will be a few days before he can get back to you.
I've never heard of an SMTP server not *sending* mail to another based upon policy/rules; I'm familiar, of course, with mail servers not *receiving* mail (eg, blocking a MX level or higher) based upon policy/rules. Could it be more likely that nabble's inbound email servers are blocking mxes.net's email?
I'll sheck with my email service support/admin folks to see what else I can find.