Is it illegal for the Bureau of Letters and Calls to investigate personal information through official letters?

Not illegal. The inquiry website shows that the bureau of letters and calls has the right to investigate personal information, so it is not illegal. The Bureau for Letters and Calls is the petition work organization of the people's government at or above the county level, and is the administrative organization responsible for the petition work of the people's government at the corresponding level. People's governments at all levels and people's governments at or above the county level shall do a good job in letters and visits, and seriously handle letters and receive visitors.