Please wash the hair before sending it.  You can use normal washing up liquid or shampoo to wash it by hand in warm water.  It may take a few washes before its clean and still may smell of your horse afterwards. I'm happy as long as it isn't caked with mud, grease or dandruff. If you've got a white tail then it may take a few more washes to get whiter.  I suggest using a non-bio wash powder and gently rubbing it in and rinsing through a couple of times.  If you have a whitening shampoo designed for whitening poo stained ponies then all the better.  I've seen people using indigo power too.