I agree with my colleague - your dog was out of your yard, but the attack claim is puzzling. What was the evidence of an attack? If someone claimed they were attacked, then there would be proof, such as photos of the injuries, medical bills, and the like. You should speak with a local attorney for further guidance.
Many if not most communities these days have pretty strict laws that require dog owners to keep their dog in their own yard, period. It sounds like your dog was indeed out of your yard, and you will have no defense to that charge. Hopefully you will just have to pay a fine.
The "attack" charge is more serious, and based on what you have written, puzzling. Why was a citation for attack issued? Did the dog charge or otherwise threaten the mailman? Is your dog a breed that is considered dangerous?
You need to know the law in your area, and the potential consequences of an attack charge. At the very least check with your local animal control for clarification on the law, and it may help to hire an attorney to represent your interests.