While we're discussing vitamin E, I want to remind everyone how critical it is to also check your horse's selenium levels. In many areas of our country, we're in selenium-deficient zones (New England is one of them), which means there isn't enough selenium in the soil, therefore there isn't enough selenium in the hay / pasture. Selenium is absolutely critical for immune system function, muscle function, thyroid function, energy levels, fighting off infections and cancerous cells, etc.
Please request that your vet check selenium AND vitamin E in bloodwork. The test is usually around $170 and worth its weight in gold. Please reach out if your horse is selenium deficient and we can discuss a plan for supplementing selenium yeast that will meet his or her needs and won't cause overload.