In addition to showing the number of retries, the message details dialog box has a recipient's field that should indicate the reason for message failure. Common reasons for failure include name resolution problems, server or router outages, mailboxes full, relaying restricted and others. Troubleshooting this problem should begin with determining the specific reason each message is failing. They may not all be failing for the same reason.
Dig Deeper on Microsoft Exchange Server Mailbox Management
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