Summary:

The following conditions can likely be the result of DNS problems:

• Messages Stuck in the Retry Queue
• Unexpected recipient failure - 571 ... Relaying denied
• Undeliverable Messages
• Messages Looping
• routing loop
• Messages sent to some users, but not others
• Mail being receive from external users, but internal to external mail not being delivered
• Cannot TELNET on PORT 25 to next SMTP relay by name
• MX Record Lookup for the Mail Server Fails
• Trace route fails to Server

Fix DNS records.