I'm queuing all mails and so store them in the database. My problem is that a mail with some French text in it failed to save properly around a simple " à ". After digging a bit through the code and searching for a solution I found this (not related to drupal but basically the same issue):
I played around with chr(194).chr(160) and got it working, unfortunately I'm on holidays now so I won't be able to provide a patch. But I don't even know where I have to replace the non-breaking space, just everywhere?