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.
i once had a cease and desist notice notice telling me to stop using an image i stole from their website (this was false as i paid a licence from the original copyright owner to use the image). I simply relied asking for proof of this, but i just got another cease and desist notice with an added bit saying they did not need to prove anything to me.
I just passed all this info onto the legitimate copyright owner who's legal team had the website of those who sent me the cease and desist notice closed down and the website owners in court.
As long as the report is genuine, I usually only oblige to C&D requests and other reports when they have proven themselves through identification of sorts, some e-mails can be spoofed to seem like they are from the genuine copyright holder after all.