What other notable achievements must European society have reached even to make this exchange of letters possible?

For the exchange of letters to occur, European society must have achieved a high level of literacy and education, enabling widespread communication and intellectual discourse. The establishment of a postal system and improvements in transportation would also be essential for the timely delivery of correspondence. Additionally, a culture that values written communication, along with the development of printing technologies, would facilitate the dissemination of ideas and information, further supporting such exchanges.