New option added in admin setting page to update Sent Mail Report  Subject/Content. For each newsletter mail and notification mail, plugin will send one report mail to admin with default content. Now with this option, admin can update that default mail content.

git-svn-id: https://plugins.svn.wordpress.org/email-subscribers/trunk@969029 b8457f37-d9ea-0310-8a92-e5e31aec5664