Have been charged for a service i did not subscribe to. company now wants me to pay outstanding dues or they will involve an international debt collection agency.

I had been receiving invoices from a newsletter distribution service for the past two months. I ignored them thinking it was spam, however today they sent me a letter of termination and asked me to settle my open invoices worth € 300 within two weeks or they would involve an international debt collector agency. Is it possible someone used my email to sign up to their service ? How should I deal with this to avoid paying and legal trouble ? This company is based in Germany and I am from India.