T-Mobile is facing criticism for sending abusive letters to its customers.

T-Mobile said it isn't behind the demeaning letters that were mailed to its customers, but it should still be held accountable.
Where did the perpetrator obtain the data?
T-Mobile has been the victim of numerous data compromises. Recently, an individual attempted to resell previously exposed data on a hacking forum. It is therefore not shocking that someone possessed names and residential addresses of T-Mobile customers.
While the FTC advises firms to have exposed information removed from websites, it is impossible to completely erase it from the internet.
T-Mobile previously resolved a class action lawsuit to settle the FCC's investigation into data breaches. The company has already paid for the errors that enabled data leaks to occur.
The source of the data may not have been T-Mobile. However, because this event concerns the information that customers entrusted to the carrier, it should still be held responsible.
T-Mobile must take this incident seriously.
T-Mobile must carefully investigate the situation and keep users informed about its progress. The letters mailed to customers appeared very professional, so it's doubtful someone went to such lengths simply to cause problems. Quick action by T-Mobile would prevent further letters.
This is important because the letters are a burden that can cause worry. The initial letter may have been a scheme to influence customers, making them more susceptible to future actions, such as an extortion attempt.