You can easily do some research on that email address. If it is a valid address, it should come up somewhere else. Look into who the brand owner is and see if it adds up.
Otherwise, I would just contact the brand owner directly, let them know you received this communication, have authentic product, follow best practices, and ask to resolve it amicably. I would leave the listing alone for now too. The easiest way to solve it, is for the brand owner to retract the complaint. If it is someone faking being a brand owner, you have bigger problems either way, because they are the brand owner in Amazon’s system it seems like, based on what you are saying. Pick and choose your battles.
1- close and delete the listing
2- contact the ip issuer - provide all the infos (invoices, ur details, supplier details, and applogise for selling on Amazon and ask for a retraction)