I was interested in this plugin but there seems to be a few errors in the code.

You are checking the $args parameter for values for cc and bcc but these do not exist in the filter (https://developer.www.ads-software.com/reference/hooks/wp_mail/).

BCC and CC can be set in the $headers element, but there is the issue that $headers can be a string or an array, so you would need to include processing for either type.