I think I would skip out on the word unregistered all together to be honest and make your mare seem like more of a treasure by adding in a little more detailed, vague context...
"Dear Mr. And Mrs.,
I was searching for a proven Welsh stallion when I came across your stud, Rosmel's Scarlet Legacy. I have an Icelandic cross mare, whom I am currently working on registering, that I would like to breed for a 2012 foal. She is a very versatile mare currently working as a ranch horse as well as showing in hunter/jumper this summer.
While looking through your website, I saw the pictures of your breeding facilities which I found both beautiful and very impressive. However, there was some information that I could not find and was hoping you could help me with. I would just like to know more about your breeding business, how you cover mares, and your policies on outside mares.
Changes I made and why:
Hello -> Dear Mr/Mrs = We all already discussed the more professional tone to this one.
Unregistered -> Using the word cross is already going to be concerning enough for more breeders, throwing in the actual word unregistered is a red flag and not advised.
2011 breeding/2012 foal -> You want to sound professional. They are already aware of the time it takes to make a foal and that a 2012 foal would have to be bred in 2011 so this is not something that really needs to be stated twice. Go with either a 2011 breeding or wanting a 2012 foal... not both.
Versatile -> one of those key words that breeders LIKE to see.
Browing -> looking = browsing leads me to believe you barely checked the website and are not serious. Looking/reading makes it look more serious overall.
Hope this helps!