Reverse DNS does not match SMTP Banner

Symptoms

Third-party tools that check for SMTP issues will report "Reverse DNS does not match SMTP Banner"  for SMTP banner checks.

Cause

By default, the Exim service will return the hostname in the SMTP Banner when receiving a valid helo. Some users may configure a different domain in the PTR record and receive this warning as a result of validation checks.

It is advised that the PTR record be updated to match the hostname to address this error. In some cases, this may not be desirable, and the workaround below can be used to customize the SMTP Banner in Exim.

Resolution

Warning: Only experienced system administrators should perform the steps provided in this article. cPanel Technical Support does not provide assistance with the steps in this tutorial. These require customizations to the Exim service that will need to be manually maintained by the system administrator for your server. 

1. Log in to WHM as the root user
2. Navigate to Home / Service Configuration / Exim Configuration Manager
3. Click the Advanced Editor tab
4. Scroll down to the smtp_banner option

5. Change the smtp_banner value to the following:

   CONFIG_TEXT: "${smtp_active_hostname} ESMTP Exim ${version_number} \#${compile_number} ${tod_full} \nWe do not authorize the use of this system to transport unsolicited, \nand/or bulk e-mail."

6. Scroll down to the Section: BEGINACL header
7. Click the Add additional configuration setting button above the Section: BEGINACL header
8. Select smtp_active_hostname from the dropdown in the line that was added

9. Add the following value for the smtp_active_hostname option:

   CONFIG_TEXT: "${if exists{/etc/custom_smtp_banner}{${lookup{$interface_address}lsearch{/etc/custom_smtp_banner}{$value}{$primary_hostname}}}{$primary_hostname}}"

10. Click the Save button at the bottom of the page
11. Log in to the server via SSH or WHM's Terminal as the root user

12. Run the following command:

    # touch /etc/custom_smtp_banner; chown root.mail /etc/custom_smtp_banner; chmod 640 /etc/custom_smtp_banner

13. Open /etc/custom_smtp_banner in your preferred text editor

14. Add entries using the $IPaddress: $domain.tld format for any IPs that use custom PTR records. For example:

    CONFIG_TEXT: 1.2.3.4: mail.domain.tld

    Note: The entries will not be automatically maintained and will require you to add or remove them manually as needed.

Additional Resources

Exim Configuration Manager | cPanel & WHM Documentation