This error show up in Exim mail logs when trying to send email from cpanel email:

2021-01-19 21:38:46 failed to expand "${lookup{$domain}cdb{/etc/domain_secondary_mx_ips.cdb}}" while checking a list: lookup of "test.com" gave DEFER: cdb: corrupt cdb file /etc/domain_secondary_mx_ips.cdb (too short).

Issue: The issue was occurring due to corruption in the /etc/domain_ secondary_mx_ips.cdb file. Check for an extra digit at the IP address.

[23:11:03 user root@94146958 ~]cPs# /scripts/dumpcdb /etc/domain_secondary_mx_ips.cdb

$VAR1 = {

      'actionforex.com' => '192.15.25.502'

     };

[/bash]

To address this issue, login and run:
# /usr/local/cpanel/bin/servers_queue queue build_secondary_mx_cache, which recreated the file with the correct IP-address information:

[/bash]

[23:30:51 user root@94146958 ~]cPs# /scripts/dumpcdb /etc/domain_secondary_mx_ips.cdb

$VAR1 = {

 'actionforex.com' => '192.15.25.50'

 };

[/bash]

Check with a test email and view of the file to ensure the correct IP address.