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Postfix mail for domain.com loops back to myself Error and Solution

I can send and receive mail using user@example.com, but system generated emails such as root@mail.example.com bounce backup with an error:

May 1 03:14:52 mail postfix/smtp[62798]: 23CA4BD6B40: to=, relay=none, delay=0.01, delays=0/0/0/0, dsn=5.4.6, status=bounced (mail for mail.example.com loops back to myself)

How do I fix this error?
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What Is The Difference Between Authentication And Authorization?

What is the difference between authentication and authorization? Why it is important to understand difference between the two? Authentication vs. Authorization -- what are they and how do they differ?
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Q. I've recently changed email user account password using passwd command, for one my postfix email server which requires SMTP authentication before sending an email. Now following error is logged into my /var/log/maillog file:

ERROR: Password not accepted from server: 535 5.7.0 Error: authentication failed: authentication failure

How do I solve this problem under Red Hat enterprise Linux running Postfix with SASL authentication?
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Linux Mail Delivery Failed With DNS Service Unavailable Error and Solution

Q. I get following error message for my postfix email server:

Mail Delivery Failed With DNS Service Unavailable Error and Solution

How do I fix this problem?
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