If you are facing an error like "not allowed extension file" while you send .eml file in emails, first you need to allow .eml file extensions as follows.

1. Login to cPanel server through SSH.
2. Copy file /etc/cpanel_exim_system_filter to /etc/cpanel_exim_system_filter_emlallowd.
3. Now, login to WHM and navigate to exim configuration. Set default exim filter file as /etc/cpanel_exim_system_filter_emlallowd.
4. Remove eml extension from cpanel_exim_system_filter_emlallowd file and Save it.