1. Delete the email and pretend that you never read it. As they say, ignorance is bliss. Okay, so this isn't the best coping mechanism. It works for a minute, though, until you start crying. (Good thing writers are never dramatic, huh?)
2. Remember that this is just one person's opinion. (Or one group of people's opinion, as it may be.) Just because they rejected it doesn't mean that it's awful and that you will never be a good writer and you are doomed to writing mediocrity all of your days.
3. Eat chocolate. Green tea also works well, although you could eat whatever you want if it makes you feel better. Exercising increases dopamine, which makes you feel happier.
4. Remember that while writing may be a big part of your life, it's not the only part of your life. So, try something different. Paint, or cook, or sleep more. Spend time with friends, or reading.
5. Accept that rejections are a part of a writer's life and hey, you might as well get used to it. Besides, you're one rejection closer to acceptance. Yay!
6. Remember that you are not your writing. You are not your writing. It's awesome that writing plays a part in your life, but it shouldn't determine your self-worth. It's okay to take a break from writing if it doesn't make you happy. You can always come back to it.
7. Read over said rejected work and see if there are any ways you could improve it.
Obviously, this isn't an exhaustive list, just tips I've gleaned from dealing with a glut of rejections in the past few weeks. If you have any other ideas, feel free to tell me in the comments!