Before I started doing my own, I used to send all my slides to Fuji using the Fuji Mailers. They did lose one roll out of about 200, but the processing and mounting was done well. I used Kodak mailers also, but the slides always came back smelling of fixer, leading me to think they weren't rinsed as well (though I now understand that rinsing E6 isn't such a sensitive thing, and may even contribute to long archival life as a sacrificial oxidant).