Sendmail shows errors when testing mail filters

Symptoms

When testing email filters from cPanel, you encounter an error similar to the following.

Error: The “/usr/sbin/sendmail -v -bF /etc/vfilters/domain.tld” command (process 3995694) reported error number 1 when it ended. 

An error similar to the following will also be in the cPanel error log.

[2022-12-01 16:00:08 -0500] warn [cpanel] Could not trace a filter for the user tvpoloni due to an error: The “/usr/sbin/sendmail -v -bF /etc/vfilters/domain.tld” command (process 3996929) reported error number 1 when it ended. LOG: MAIN PANIC DIE
  failed to expand "${if exists{/etc/custom_smtp_banner}{${lookup{$interface_address}lsearch{/etc/custom_smtp_banner}{$value}{$primary_hostname}}}{$primary_hostname}}" (smtp_active_hostname): failed to open /etc/custom_smtp_banner for linear search: Permission denied (euid=1027 egid=979)

Description

The error occurs when the /etc/custom_smtp_banner file has incorrect permissions. The permissions should be set to 644.

Workaround

1. Access the server's command line as the 'root' user via SSH or "Terminal" in WHM.
2. Set the /etc/custom_smtp_banner permissions to 644.

   chmod 644 /etc/custom_smtp_banner