Look in the bounced message for any mention of verizon.net and an associated 550 error code. Examples of actual error messages are below.
host relay.verizon.net[206.46.232.11] said: 550 You are not allowed to send mail:sv22pub.verizon.net
host relay.verizon.net [206.46.232.11]: 550 You are not allowed to send mail:sv24pub.verizon.net
This document is: http://faq.rahul.net/cgi-bin/fom.cgi?file=61
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This is a Faq-O-Matic 2.721.