Gw / GMS mail from windows phone - encoding characters

We have a groupwise 2012.0.3 server with GMS 2.0.
When we send an email from a windows phone (8 or 8.1), this message is well in groupwise client, but not in outlook client.
The special french characters (é, ù, à, etc...) are tranformed to chinese characters in outlook 2010 client, or "?" when we use thunderbird (last version).

So, I think there is a mime problem in gms, but how can I resolve it? (maybe it's not that!?)

Can you help me?