All legal complaints go directly to a lawyer. Don't try to discern the information or try to respond. This is a legal proceeding and it's best left to someone who has knowledge in the field.
If you're referring to a cease and desist notice, it's entirely possible that the person who sent it doesn't have the legal stand to actually enforce it. It's best to have someone that understands the law read the information and respond guide as necessary.
If the issue is with one of your clients, then the notice is forwarded to the client and they must respond within "X" hours/days or the website is suspended until resolved. This is something we used to have to do in the case of DMCA notices. The steps of what you will do when you receive complaints or notices must be clearly outlined in your terms of service or usage policy also.