What to do if the email sending of Debian mail server fails

When the Debian mail server mail sending fails, you can follow the following steps to troubleshoot and resolve:

Check the mail server configuration

• Check the configuration file : Double-check the configuration file of the mail server, such as /etc/exim4/update-exim4.conf.conf, to ensure that the settings are correct.
• DNS resolution : Confirm that DNS resolution is normal, you can pass the telnet smtp.qq.com 25 command to test.
• Port Settings : Make sure that the SMTP port (usually 25) is open and not blocked by the firewall.

Check the email send log

• View log files : Check the log files of the mail server, such as /var/log/mail.log, for detailed error information.

Check the email blacklist

• Check the blacklist status : Confirm whether the email server is blacklisted, such as RBL (real-time black hole list), and process it as needed.

Check email size limit

• Resize emails : If the email is too large, it may cause the sending to fail. You can try to reduce the size of your email attachment.

Check the mail queue

• Clean up the email queue : If the email queue is blocked, it may affect the sending of the email. You can use the exim4 -bp command to view the queue and clean it up.

Check security policies

• Update security policy : Ensure that the security policies of the mail server such as SPF, DMARC, etc. are set correctly to allow legal mail to pass.

Restart the mail service

• Restart the service : After modifying the configuration file, make sure to restart the mail service for the changes to take effect. For example, use systemctl restart exim4.

Through the above steps, the problem of mail sending failure of Debian mail server can usually be solved. If the problem persists, it is recommended to view specific error information and seek help in the relevant community or forum.

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