problem with encoding for the "from" sender address

Issue Summary

I get the following sender information in newsletter I create :

From: =?UTF-8?B?Ij0/VVRGLTg/Qj9VMlZ5ZG1salpTQmpkV3gwZFhKbGJDQXRJRlZ1YVhabGNuTnA=?=.=?UTF-8?B?ZE1PcElGSmxibTVsY3lBeUlDMGdTR0YxZEdVPT89CiA9P1VURi04P0I/SUVKeVo=?=.=?UTF-8?B?WFJoWjI1bD89IiA8d2VibWFzdGVyQHVoYi5mcj4=?=@test.toto.fr
Content-Type: multipart/alternative;
charset=utf-8;
boundary="837b4ad8958100a6003ee97afb00422e"

This problem only occurs when I create a new content. When I send an simplenews with a content that already exist before I activate the simplenews module, the from sender is the correctly encode, and the from address is nice.
This problem should then not to be due to a server encoding problem as mentioned in #326746: Verry strange sender information ... --> From: =?UTF-8?B?Ij0/VVRGLTg ....

Does somebody have an idea how to fix this?