Just ignore it, you don't have to give them anything they ask.. And they also can't prove you even got or read the email... well unless you replied of course
But even if you replied, it still doesn't 100% prove it was you. Anyone can sit at your PC.
Most likely it's bullshit as email is not the official way of contact - but I know a case when a police officer phoned a dude to ask him to come and testify. It is faster for them, but you can ask for the official mail.