If you use custom headers, it will not replace the core functionality of generating TO: mail headers. The functionality has to be customized to put names in the TO: headers, but I am afraid 99% of the users would never have first and last names or no need to add the names here. However, we can add it as an additional optional feature in Mumara Campaigns. We are in the middle of launching PHP 7 edition of Mumara Classic, I'll check if it's something our project board can approve.