Hello everyone, I have Microsoft Exchange 2007 installed on Microsoft Windows Server 2003. Roles installed: Hub Transport, Client Access, Mailbox Problem: Mail messages fromour serverto some spesific servers are not delivered (for example Verizon.net). They stay in queue, and system retries to send them, and after 2 days on unsuccessful trying, it expires them.For such messages, an NDR like this is sent to the sender user: From: Microsoft ExchangeSent: Monday, December 03, 2007 12:01 PMTo: username@verizon.netSubject: Undeliverable: message subject Delivery has failed to these recipients or distribution lists: username@verizon.netMicrosoft Exchange has been trying to deliver this message without success and has stopped trying. Please try sending this message again, or provide the following diagnostic text to your system administrator. We asked Verizon about this and they told us that our server is not blacklisted. When looking into message queue, I found out that there are some messages with unknown sender (it shows sender like <> ) to different addresses in other servers. In other words, sender and receiver are unknown to me. And subjects are like subjects of Spam messages and I'm sure that they are not sent from users of our mail system. And the SCL for these messages is -1. I would be really thankful if someone can help and guide me about this. Thanks, Simin