poison letter ,,,,,,,,, mmmmmmmmm do a highly secret super duper confidential enquiry on that letter and its contents, it must just be hoax or truth also and might be the person sending email doesnt want to identify themselves because of security reasons. Just imagine when police gets call from anonymous person saying there is bomb at this place, do they just ignore it because the caller is unknown? No ,,,
so just check the validity of the letter and its truthness if found true u can take action against dept manager if not then file cyber crime case against the person sending spam mails,