In my project I've needed customize the messages sended to new subscribers and unsubscribers of each newsletter, to do this, I've modified the configuration interface of the neswletters adding the fields originally present in the module configuration form and I've added new configuration variables for each newsletter.

I attach the patches with my changes, maybe it'll be usefull for you

CommentFileSizeAuthor
simplenews.subscription.inc_.patch1.3 KBfcasanellas
simplenews.module.patch4.77 KBfcasanellas
simplenews.admin_.inc_.patch8.96 KBfcasanellas