On the face of it, monitoring your child's e-mail sounds like an invasion of privacy, especially if you have a teen with special needs. Obviously, this will not be an acceptable set-up for all kids and parents. However, if your child is vulnerable to cyberbullying, fearful of unwanted correspondence, or susceptible to bad decisions, it may be agreeable to both of you to have protective parental eyes on any messages that move through. Set up Facebook and other social-networking tools to send alerts to this address, and you can also keep an eye on goings-on there.
Talk with your child about what you want to do and why. If this is the only condition in which you will allow e-mail, discuss that. Then, when you're in agreement, move on to the set-up.