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We use iis 6 to send out emails for our website but some email addresses bouce backs. It only appears to be 2 addresses. There is at least 100 different email addresses sent to at the same time as these. Here are the 2 different messages we get.
You do not have permission to send to this recipient. For assistance, contact your system administrator. secure.example.com #5.7.1 smtp;550 5.7.1 Fix reverse DNS for xx.xx.xxx.xxx or use your ISP's mail server
Could not deliver the message in the time limit specified. Please retry or contact your administrator. secure.example.com #4.4.7
Not sure if they both relate to reverse dns. If you can help or require more information please let me know.