Re: Simple solution
Anything that is serious such as notice of a copyright infringement shouldn't be coming through a gmail account. Certainly not a notice from an attorney.
When a client hands me a gmail address, they don't get the same level of service as I've learned from experience that those people are the least likely to be around for the long term. Somebody with their own domain and even the most simplistic web site gets a lot more cred. I know that my website doesn't bring in very many clients, but it does act as a flyer that shows the services I provide and samples of my work. If I handed out the same thing on paper, it would get thrown away as nobody is filing tangible handouts, catalogs, etc. I used to have a couple of cabinets of catalogs and cut sheets and have long since turned that into bookmarks, a database and spreadsheets.
Flag number one is always an email address from a free service. This would be especially so for a copyright infringement notice. If the entity couldn't be bothered to have a proper business backend, I can ignore them as it's very unlikely they'll take the matter any further. If they do, I expect I'd be getting notice from a verifiable attorney.