I immediately informed T-Systrms via their crappy contact form on their crappy website. After a couple of days I received an e-mail which I could not reply to, asking me to provide more proof that I was receiving someone else's emails through. It also said I can't reply directly to the email and I had to provide this info via their crappy contact form on their crappy website.
I refused to do so, since I had already given them the name, address, and customer id of their new customer and it seemed that the procedure is going to be unnecesarily time consuming.
After that I received emails about tracking of a package containing the hardware for the internet connection. Then I received an email that nobody was home to pick up the delivery and when the next attempt will be. UAnd utimately that the package was going to ve returned. After that I guess the customer got in touch with T-Systems and fixed the email error.
Nobody from T-Systems contacted me asking to destroy the documents and emails containing someone else's personal data.