It depends upon the request. It sounds like you received a remedial.
Some remedials are okay to upload as an EMail, some require a signature (or signatures) and mailed through USPS.
The remedial in question should state as to how they want to receive an answer to it.
Typically, the snail-mail goes to:
Department of Defense
Medical Examination Review Board (DoDMERB)
8034 Edgerton Dr. Suite 132
USAF Academy, Colorado 80840-2200
Don't just send it there, though. Check the remedial and find out how they want to receive the answer. If you can't figure it out, ask your Admissions Counselor in a quick EMail, with the remedial number.
The Service Academy you're applying to makes no difference, as the USAF is the clearing house for all things Medical Review-related. So if it has to be mailed, it will probably be to the above address.
My DS emailed his statement in the body of his email to the clerical support staff email that was listed at the bottom of the letter he received in his DoDMERB portal. His DoDMERB status in his USMA portal has been changed to "Qualified"